Method for fabricating bulk acoustic wave resonator with mass adjustment structure

ABSTRACT

A method for fabricating bulk acoustic wave resonator with mass adjustment structure, comprising following steps of: forming a sacrificial structure mesa on a substrate; etching the sacrificial structure mesa such that any two adjacent parts have different heights, a top surface of a highest part of the sacrificial structure mesa is coincident with a mesa top extending plane; forming an insulating layer on the sacrificial structure mesa and the substrate; polishing the insulating layer to form a polished surface; forming a bulk acoustic wave resonance structure including a top electrode, a piezoelectric layer and a bottom electrode on the polished surface; etching the sacrificial structure mesa to form a cavity; the insulating layer between the polished surface and the mesa top extending plane forms a frequency tuning structure, the insulating layer between the mesa top extending plane and the cavity forms a mass adjustment structure.

CROSS-REFERENCE TO RELATED DOCUMENTS

The present invention is a continuation in part (CIP) to a U.S. patentapplication Ser. No. 15/477,758 entitled “BULK ACOUSTIC WAVE FILTER ANDA METHOD OF FREQUENCY TUNING FOR BULK ACOUSTIC WAVE RESONATOR OF BULKACOUSTIC WAVE FILTER” filed on Apr. 3, 2017.

FIELD OF THE INVENTION

The present invention is related to a method for fabricating bulkacoustic wave resonator with mass adjustment structure, especially amethod for fabricating bulk acoustic wave resonator with mass adjustmentstructure and frequency tuning structure.

BACKGROUND OF THE INVENTION

Please refer to FIG. 8, which is a schematic view of an embodiment of abulk acoustic wave resonator of the conventional technology. Theresonator comprises a substrate 90, a bottom electrode 91, apiezoelectric layer 92, a top electrode 93, a cavity 94 and an annularpiezoelectric layer recess 95. The bottom electrode 91 is formed on thesubstrate 90. The piezoelectric layer 92 is formed on the bottomelectrode 91. The top electrode 93 is formed on the piezoelectric layer92. The cavity 94 is formed under the bottom electrode 91 on thesubstrate 90. The overlapping of the top electrode 93, the piezoelectriclayer 92 and the bottom electrode 91 form a resonance film of the bulkacoustic wave resonator. The annular piezoelectric layer recess 95 isformed by removing the material of the piezoelectric layer 92 around theperiphery of the resonance film of the bulk acoustic wave resonator. Byforming the annular piezoelectric layer recess 95, the boundarycondition of the periphery of the resonance film of the bulk acousticwave resonator is changed. Since the boundary condition of the peripheryof the resonance film of the bulk acoustic wave resonator is changed,the ratio of the reflection acoustic wave and the incident acoustic waveis also changed when then incident acoustic wave reflects at theperiphery of the resonance film of the bulk acoustic wave resonator. Bydesigning and adjusting a proper width and a proper depth of the annularpiezoelectric layer recess 95, the ratio of the reflection acoustic waveand the incident acoustic wave may be adjusted such that the Q factor ofthe bulk acoustic wave resonator is enhanced.

Since a width of the resonance film of the bulk acoustic wave resonatoris usually much greater than a depth of the cavity 94, and furthermorethe resonance film of the bulk acoustic wave resonator is formed by thetop electrode 93, the piezoelectric layer 92 and the bottom electrode91, especially the top electrode 93 and the bottom electrode 91 areformed by metal, therefore, the resonance film of the bulk acoustic waveresonator may be bended downwardly when affected by stress. Hence, abottom of the bottom electrode 91 is possibly touched with the substrate90 (a bottom of the cavity 94) such that the characteristics of the bulkacoustic wave resonator may be affected. Removing the material of thepiezoelectric layer 92 around the periphery of the resonance film of thebulk acoustic wave resonator to form the annular piezoelectric layerrecess 95 may affect the mechanical structure strength of the resonancefilm of the bulk acoustic wave resonator such that the resonance film ofthe bulk acoustic wave resonator become more easily bended downwardlywhen affected by stress. Moreover, the insufficient mechanical strengthof the resonance film of the bulk acoustic wave resonator may evenresult in collapse of the resonance film of the bulk acoustic waveresonator.

Since the acoustic wave is propagating and resonating in the resonancefilm of the bulk acoustic wave resonator, therefore whether the entireflatness of the top electrode 93, the piezoelectric layer 92 and thebottom electrode 91 of the resonance film of the bulk acoustic waveresonator may directly affect the characteristics of the bulk acousticwave resonator. In another embodiment of a bulk acoustic wave resonatorof the conventional technology, a protruding structure is formed alongan edge of a top surface of the bottom electrode 91. By forming theprotruding structure, the boundary condition of the periphery of theresonance film of the bulk acoustic wave resonator is changed such thatthe ratio of the reflection acoustic wave and the incident acoustic waveis also changed. By designing and adjusting a proper dimension of theprotruding structure, the ratio of the reflection acoustic wave and theincident acoustic wave may be adjusted such that the Q factor of thebulk acoustic wave resonator is enhanced. However forming the protrudingstructure along the edge of the top surface of the bottom electrode 91,the flatness of the piezoelectric layer 92 may be poor such that theentire flatness of the resonance film of the bulk acoustic waveresonator is affected. Hence, the characteristics of the acoustic wavepropagating in the resonance film of the bulk acoustic wave resonatormay be affected so that the characteristics of the bulk acoustic waveresonator are adversely affected.

Accordingly, the present invention has developed a new design which mayavoid the above mentioned drawbacks, may significantly enhance theperformance of the devices and may take into account economicconsiderations. Therefore, the present invention then has been invented.

SUMMARY OF THE INVENTION

The main technical problems that the present invention is seeking tosolve are to increase the mechanical strength of the resonance film ofthe bulk acoustic wave resonator, to avoid affecting the entire flatnessof the resonance film of the bulk acoustic wave resonator, and, in themean while, suppress the spurious mode of the bulk acoustic waveresonator.

In order to solve the problems mentioned the above and to achieve theexpected effect, the present invention provides a method for fabricatingbulk acoustic wave resonator with mass adjustment structure, comprisingfollowing steps of: Step D1: forming a sacrificial structure mesa on asubstrate, wherein the sacrificial structure mesa is divided into aplurality of parts; Step D2: etching the sacrificial structure mesa suchthat any two adjacent parts of the sacrificial structure mesa havedifferent heights, wherein a highest part of the sacrificial structuremesa has a highest mesa top surface, wherein a mesa top extending planeis coincident with the highest mesa top surface; Step D3: forming aninsulating layer on the sacrificial structure mesa and the substrate;Step D4: polishing the insulating layer by a chemical-mechanicalplanarization process to form a polished surface; Step D5: forming abulk acoustic wave resonance structure on the polished surface, whereinthe bulk acoustic wave resonance structure is located above thesacrificial structure mesa, wherein the Step D5 comprises followingsteps of: Step D51: forming a bottom electrode layer on the polishedsurface; Step D52: forming a piezoelectric layer on the bottom electrodelayer; and Step D53: forming a top electrode layer on the piezoelectriclayer; and Step D6: etching the sacrificial structure mesa to form acavity, wherein the cavity is located under the bulk acoustic waveresonance structure; wherein, in the Step D4, (1) the insulating layeris polished such that the sacrificial structure mesa is not exposed,wherein the insulating layer under the bulk acoustic wave resonancestructure, above the cavity, and between the polished surface and themesa top extending plane forms a frequency tuning structure, wherein theinsulating layer under the bulk acoustic wave resonance structure andbetween the mesa top extending plane and the cavity forms a massadjustment structure; or (2) the insulating layer is polished such thatthe sacrificial structure mesa is exposed, wherein the insulating layerunder the bulk acoustic wave resonance structure and between thepolished surface and the cavity forms a mass adjustment structure.

In an embodiment, after the Step D4, the plurality of parts of thesacrificial structure mesa has a geometric configuration; wherein thegeometric configuration of the sacrificial structure mesa is correlatedto a geometric configuration of the mass adjustment structure; therebyby adjusting the geometric configuration of the sacrificial structuremesa, the geometric configuration of the mass adjustment structure isadjusted such that a Q factor of the bulk acoustic wave resonator isenhanced.

In an embodiment, the substrate is a semiconductor substrate; whereinthe sacrificial structure mesa is made of at least one material selectedfrom the group consisting of: metal, alloy and epitaxial structure.

In an embodiment, the substrate is a compound semiconductor substrate;wherein the Step D1 comprises following steps of: Step D1: forming asacrificial structure on the substrate; and Step D12: etching thesacrificial structure to form the sacrificial structure mesa.

In an embodiment, (1) the substrate is made of GaAs; the sacrificialstructure comprises a GaAs layer; or (2) the substrate is made of InP;the sacrificial structure comprises an InGaAs layer.

In an embodiment, further comprises a following step of: forming abottom etching stop layer on the substrate, wherein the sacrificialstructure is formed on the bottom etching stop layer; wherein (1) thebottom etching stop layer is made of InGaP; or (2) the bottom etchingstop layer is made of InP.

In addition, the present invention further provides a method forfabricating bulk acoustic wave resonator with mass adjustment structure,comprising following steps of: Step E1: forming a sacrificial structuremesa on a substrate; Step E2: forming an insulating layer on thesacrificial structure mesa and the substrate; Step E3: polishing theinsulating layer by a prior chemical-mechanical planarization process toform a pre-polished surface, such that the sacrificial structure mesa isexposed, wherein the sacrificial structure mesa is divided into aplurality of parts; Step E4: etching the sacrificial structure mesa suchthat any two adjacent parts of the sacrificial structure mesa havedifferent heights, wherein a highest part of the sacrificial structuremesa has a highest mesa top surface, wherein a mesa top extending planeis coincident with the highest mesa top surface; Step E5: forming a bulkacoustic wave resonance structure, wherein the bulk acoustic waveresonance structure is located above the sacrificial structure mesa,wherein the Step E5 comprises following steps of: Step E51: forming asecond polish layer on the sacrificial structure mesa and the insulatinglayer, wherein the second polish layer is made of insulator; Step E52:polishing the second polish layer by a chemical-mechanical planarizationprocess to form a polished surface such that the sacrificial structuremesa is not exposed; Step E53: forming a bottom electrode layer on thepolished surface; Step E54: forming a piezoelectric layer on the bottomelectrode layer; and Step E55: forming a top electrode layer on thepiezoelectric layer; and Step E6: etching the sacrificial structure mesato form a cavity, wherein the cavity is located under the bulk acousticwave resonance structure; wherein the second polish layer under the bulkacoustic wave resonance structure, above the cavity, and between thepolished surface and the mesa top extending plane forms a frequencytuning structure, wherein the second polish layer under the bulkacoustic wave resonance structure and between the mesa top extendingplane and the cavity forms a mass adjustment structure.

In an embodiment, after the Step E52, the plurality of parts of thesacrificial structure mesa has a geometric configuration; wherein thegeometric configuration of the sacrificial structure mesa is correlatedto a geometric configuration of the mass adjustment structure; therebyby adjusting the geometric configuration of the sacrificial structuremesa, the geometric configuration of the mass adjustment structure isadjusted such that a Q factor of the bulk acoustic wave resonator isenhanced.

In an embodiment, the substrate is a semiconductor substrate; whereinthe sacrificial structure mesa is made of at least one material selectedfrom the group consisting of: metal, alloy and epitaxial structure.

In an embodiment, the substrate is a compound semiconductor substrate;wherein the Step E1 comprises following steps of: Step E11: forming asacrificial structure on the substrate; and Step E12: etching thesacrificial structure to form the sacrificial structure mesa.

In an embodiment, (1) the substrate is made of GaAs; the sacrificialstructure comprises a GaAs layer; or (2) the substrate is made of InP;the sacrificial structure comprises an InGaAs layer.

In an embodiment, further comprises a following step of: forming abottom etching stop layer on the substrate, wherein the sacrificialstructure is formed on the bottom etching stop layer; wherein (1) thebottom etching stop layer is made of InGaP; or (2) the bottom etchingstop layer is made of InP.

In addition, the present invention further provides a method forfabricating bulk acoustic wave resonator with mass adjustment structure,comprising following steps of: Step F1: forming a sacrificial structuremesa on a substrate; Step F2: forming an insulating layer on thesacrificial structure mesa and the substrate; Step F3: polishing theinsulating layer by a prior chemical-mechanical planarization process toform a pre-polished surface, such that the sacrificial structure mesa isexposed, wherein the sacrificial structure mesa is divided into aplurality of parts; Step F4: etching the sacrificial structure mesa suchthat any two adjacent parts of the sacrificial structure mesa havedifferent heights, wherein a highest part of the sacrificial structuremesa has a highest mesa top surface, wherein a mesa top extending planeis coincident with the highest mesa top surface; Step F5: forming a bulkacoustic wave resonance structure, wherein the bulk acoustic waveresonance structure is located above the sacrificial structure mesa,wherein the Step F5 comprises following steps of: Step F51: forming asecond polish layer on the sacrificial structure mesa and the insulatinglayer, wherein the second polish layer is made of at least one materialselected from the group consisting of: metal and alloy; Step F52:polishing the second polish layer by a chemical-mechanical planarizationprocess to form a polished surface such that the sacrificial structuremesa is not exposed; Step F53: patterning the second polish layer; StepF54: forming a piezoelectric layer on the polished surface of the secondpolish layer and the pre-polished surface of the insulating layer; andStep F55: forming a top electrode layer on the piezoelectric layer; andStep F6: etching the sacrificial structure mesa to form a cavity,wherein the cavity is located under the bulk acoustic wave resonancestructure; wherein the second polish layer under the piezoelectriclayer, above the cavity, and between the polished surface and the mesatop extending plane forms a bottom electrode layer of the bulk acousticwave resonance structure; wherein the second polish layer under the bulkacoustic wave resonance structure and between the mesa top extendingplane and the cavity forms a mass adjustment structure.

In an embodiment, after the Step F52, the plurality of parts of thesacrificial structure mesa has a geometric configuration; wherein thegeometric configuration of the sacrificial structure mesa is correlatedto a geometric configuration of the mass adjustment structure; therebyby adjusting the geometric configuration of the sacrificial structuremesa, the geometric configuration of the mass adjustment structure isadjusted such that a Q factor of the bulk acoustic wave resonator isenhanced.

In an embodiment, the substrate is a semiconductor substrate; whereinthe sacrificial structure mesa is made of at least one material selectedfrom the group consisting of: metal, alloy and epitaxial structure.

In an embodiment, the substrate is a compound semiconductor substrate;wherein the Step F1 comprises following steps of: Step F11: forming asacrificial structure on the substrate; and Step F12: etching thesacrificial structure to form the sacrificial structure mesa.

In an embodiment, (1) the substrate is made of GaAs; the sacrificialstructure comprises a GaAs layer; or (2) the substrate is made of InP;the sacrificial structure comprises an InGaAs layer.

In an embodiment, further comprises a following step of: forming abottom etching stop layer on the substrate, wherein the sacrificialstructure is formed on the bottom etching stop layer wherein (1) thebottom etching stop layer is made of InGaP; or (2) the bottom etchingstop layer is made of InP.

For further understanding the characteristics and effects of the presentinvention, some preferred embodiments referred to drawings are in detaildescribed as follows.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A˜1F are the cross-sectional schematics showing steps of anembodiment of a method for forming cavity of bulk acoustic waveresonator of the present invention.

FIGS. 1G and 1H are the cross-sectional schematics showing steps ofanother embodiment of a method for forming cavity of bulk acoustic waveresonator of the present invention.

FIG. 1I is the cross-sectional schematic showing an epitaxial structureof an embodiment of a method for forming cavity of bulk acoustic waveresonator of the present invention.

FIGS. 1J and 1K are the cross-sectional schematics showing steps ofanother embodiment of a method for forming cavity of bulk acoustic waveresonator of the present invention.

FIG. 1L is the cross-sectional schematic showing an embodiment of amethod for forming cavity of bulk acoustic wave resonator of the presentinvention.

FIGS. 2A˜2F are the cross-sectional schematics showing steps of anembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 2G and 2H are the cross-sectional schematics showing steps ofanother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIGS. 2I and 2J are the cross-sectional schematics showing twoembodiments of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 2K˜2N are the cross-sectional schematics showing steps of anotherembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 3A˜3G are the cross-sectional schematics showing steps of anembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 3H and 3I are the cross-sectional schematics showing steps ofanother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIGS. 3J and 3K are the cross-sectional schematics showing steps ofanother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIG. 3L is the cross-sectional schematic showing another embodiment of amethod of frequency tuning for bulk acoustic wave resonator of bulkacoustic wave filter of the present invention.

FIGS. 4A˜4D are the cross-sectional schematics showing steps of anembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 4E and 4F are the cross-sectional schematics showing steps ofanother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIGS. 4G and 4H are the cross-sectional schematics showing steps ofanother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIG. 4I is the cross-sectional schematic showing another embodiment of amethod of frequency tuning for bulk acoustic wave resonator of bulkacoustic wave filter of the present invention.

FIGS. 4J˜4M are the cross-sectional schematics showing steps of anotherembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 5A˜5C are the cross-sectional schematics showing steps of anembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIG. 5D is the cross-sectional schematic showing another embodiment of amethod of frequency tuning for bulk acoustic wave resonator of bulkacoustic wave filter of the present invention.

FIGS. 5E˜5G are the cross-sectional schematics showing steps of anotherembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 5H˜5K are the cross-sectional schematics showing steps of anotherembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 5L and 5M are the cross-sectional schematics showing steps ofanother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIGS. 5N and 5O are the cross-sectional schematics showing steps ofanother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIG. 5P is the cross-sectional schematic showing another embodiment of amethod of frequency tuning for bulk acoustic wave resonator of bulkacoustic wave filter of the present invention.

FIGS. 6A˜6C are the cross-sectional schematics showing steps of anembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIGS. 6D˜6F are the cross-sectional schematics showing three embodimentsof a method of frequency tuning for bulk acoustic wave resonator of bulkacoustic wave filter of the present invention.

FIG. 6G is the partial enlarged cross-sectional schematic showing anembodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention.

FIG. 6H is the partial enlarged cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.

FIGS. 7A˜7C are the cross-sectional schematics showing steps of anembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention.

FIGS. 7D˜7E are the cross-sectional schematics showing steps of anotherembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention.

FIGS. 7F˜7H are the cross-sectional schematics showing steps of anembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention.

FIGS. 7I˜7K are the cross-sectional schematics showing steps of anotherembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention.

FIGS. 7L˜7M are the cross-sectional schematics showing steps of anembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention.

FIGS. 7N˜7O are the cross-sectional schematics showing steps of anotherembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention.

FIGS. 7P˜7Q are the cross-sectional schematics showing steps of anotherembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention.

FIG. 7R is the cross-sectional schematic showing an embodiment of amethod for fabricating bulk acoustic wave resonator with mass adjustmentstructure of the present invention.

FIG. 7S is the electrode top-view shape of an embodiment of a method forfabricating bulk acoustic wave resonator with mass adjustment structureof the present invention.

FIG. 8 is a schematic view of an embodiment of a bulk acoustic waveresonator of the conventional technology.

DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENTS

Please refer to FIGS. 1A 1F, which are the cross-sectional schematicsshowing steps of an embodiment of a method for forming cavity of bulkacoustic wave resonator of the present invention. A method for formingcavity of bulk acoustic wave resonator of the present inventioncomprises following steps of: Step A1: (please referring to FIG. 1B)forming a sacrificial epitaxial structure mesa 60 on a compoundsemiconductor substrate 13, which comprises following steps of: (pleasereferring to FIG. 1A) forming an sacrificial epitaxial structure 28 onthe compound semiconductor substrate 13; and (please referring to FIG.1B) etching the sacrificial epitaxial structure 28 to form thesacrificial epitaxial structure mesa 60; Step A2: (please referring toFIG. 1C) forming an insulating layer 11 on the sacrificial epitaxialstructure mesa 60 and the compound semiconductor substrate 13, whereinthe insulating layer 11 is made of at least one material selected fromthe group consisting of: silicon nitride (SiN_(x)), silicon oxide (SiO₂)and polymer; Step A3: (please referring to FIG. 1D) polishing theinsulating layer 11 by a chemical-mechanical planarization process toform a polished surface 41; Step A4: (please referring to FIG. 1E)forming a bulk acoustic wave resonance structure 3 on the polishedsurface 41, wherein the bulk acoustic wave resonance structure 3 islocated above the sacrificial epitaxial structure mesa 60, wherein theStep A4 comprises following steps of: Step A41: forming a bottomelectrode layer 30 on the polished surface 41; Step A42: forming apiezoelectric layer 31 on the bottom electrode layer 30; and Step A43:forming a top electrode layer 32 on the piezoelectric layer 31; and StepA5: (please referring to FIG. 1F) etching the sacrificial epitaxialstructure mesa 60 to form a cavity 40, wherein the cavity 40 is locatedunder the bulk acoustic wave resonance structure 3. In the Step A3, theinsulating layer 11 is polished such that the sacrificial epitaxialstructure mesa 60 is not exposed, wherein the insulating layer 11between the bottom electrode layer 30 and the sacrificial epitaxialstructure mesa 60 forms a frequency tuning structure 50, wherein thefrequency tuning structure 50 has a thickness T, the bulk acoustic waveresonance structure 3 has a resonance frequency F, thereby adjusting thethickness T of the frequency tuning structure 50 is capable of tuningthe resonance frequency F of the bulk acoustic wave resonance structure3. As the thickness T of the frequency tuning structure 50 is increased,the resonance frequency F of the bulk acoustic wave resonance structure3 is decreased; while as the thickness T of the frequency tuningstructure 50 is decreased, the resonance frequency F of the bulkacoustic wave resonance structure 3 is increased. The main feature of amethod for forming cavity of bulk acoustic wave resonator of the presentinvention is to use the compound semiconductor substrate 13 to form thesacrificial epitaxial structure 28 as the sacrificial layer and thenpolish the insulating layer 11 by a chemical-mechanical planarizationprocess. The advantage is that it helps to precisely adjust thethickness T of the frequency tuning structure 50, and it also helps toprecisely tune the frequency F of the bulk acoustic wave resonancestructure 3. However if the thickness T of the frequency tuningstructure 50 is too thick, then it will affect the resonance state ofthe bulk acoustic wave resonance structure 3. Hence, the thickness T ofthe frequency tuning structure 50 is required to be less than 1000 nm.In some preferable embodiments, the thickness T of the frequency tuningstructure 50 is equal to or less than 300 nm.

Please refer to FIGS. 1G and 1H, which are the cross-sectionalschematics showing steps of another embodiment of a method for formingcavity of bulk acoustic wave resonator of the present invention. Themain steps for forming the embodiment shown in FIG. 1H are basically thesame as those for forming the embodiment shown in FIG. 1F, except thatin the Step A3, the insulating layer 11 is polished such that thesacrificial epitaxial structure mesa 60 is exposed (please referring toFIG. 1G); then forming the bulk acoustic wave resonance structure 3 onthe polished surface 41 and etching the sacrificial epitaxial structuremesa 60 to form the cavity 40 (please referring to FIG. 1H). In currentembodiment, the bulk acoustic wave resonance structure 3 does not havethe frequency tuning structure 50 as the embodiment shown in FIG. 1F.

Please refer to FIG. 1I, which is the cross-sectional schematic showingan epitaxial structure of an embodiment of a method for forming cavityof bulk acoustic wave resonator of the present invention. The mainstructure of the epitaxial structure of the embodiment in FIG. 1I isbasically the same as that of the embodiment in FIG. 1A, except that anetching protection layer 12 is further formed on a bottom surface of thecompound semiconductor substrate 13. The function of the etchingprotection layer 12 is to protect the bottom surface of the compoundsemiconductor substrate 13 so as to prevent the bottom surface of thecompound semiconductor substrate 13 from damage of being etched duringthe etching process of the fabricating process (especially being etchedby the etchant of the wet etching). The etching protection layer 12 ismade of at least one material selected from the group consisting of:silicon nitride (SiN_(x)), silicon oxide (SiO₂), aluminum nitride (AlN)and photoresist. The preferable material for the etching protectionlayer 12 is silicon nitride. Usually the etching protection layer 12will be removed after the Step A5 in order to perform a substratethinning process. In all other embodiments of the present invention, nomatter the substrate is a semiconductor substrate or a compoundsemiconductor substrate, the etching protection layer 12 can be appliedto be formed on the bottom surface of the semiconductor substrate or thecompound semiconductor substrate to protect the bottom surface of thesubstrate.

Please refer to FIGS. 1J and 1K, which are the cross-sectionalschematics showing steps of another embodiment of a method for formingcavity of bulk acoustic wave resonator of the present invention. Themain structure of the epitaxial structure of the embodiment in FIG. 1Jis basically the same as that of the embodiment in FIG. 1A, except thata bottom etching stop layer 20 is further formed on the compoundsemiconductor substrate 13, wherein the sacrificial epitaxial structure28 is formed on the bottom etching stop layer 20. When etching thesacrificial epitaxial structure 28 to form the sacrificial epitaxialstructure mesa 60, the sacrificial epitaxial structure 28 around thesacrificial epitaxial structure mesa 60 is etched and the etching stopsat the bottom etching stop layer 20. Under the sacrificial epitaxialstructure mesa 60 is the bottom etching stop layer 20. The embodiment ofthe bulk acoustic wave resonator shown in FIG. 1K is fabricated from theepitaxial structure of the embodiment shown in FIG. 1J. The mainstructure of the embodiment in FIG. 1K is basically the same as that ofthe embodiment in FIG. 1F, except that a bottom etching stop layer 20 isfurther formed on the compound semiconductor substrate 13. In the StepA2, the insulating layer 11 is formed on the sacrificial epitaxialstructure mesa 60 and the bottom etching stop layer 20. Therefore, afterthe sacrificial epitaxial structure mesa 60 is etched in the Step A5,the cavity 40 is formed above the bottom etching stop layer 20. In someembodiments, the compound semiconductor substrate 13 is made of GaAs;the sacrificial epitaxial structure 28 comprises a sacrificial epitaxiallayer; the sacrificial epitaxial layer is made of GaAs; wherein thesacrificial epitaxial layer has a thickness between 50 nm and 5000 nm;the bottom etching stop layer 20 is made of InGaP; wherein the bottometching stop layer 20 has a thickness between 20 nm and 500 nm. In someother embodiments, the compound semiconductor substrate 13 is made ofInP; the sacrificial epitaxial structure 28 comprises a sacrificialepitaxial layer; the sacrificial epitaxial layer is made of InGaAs;wherein the sacrificial epitaxial layer has a thickness between 50 nmand 5000 nm; the bottom etching stop layer 20 is made of InP; whereinthe bottom etching stop layer 20 has a thickness between 20 nm and 500nm.

Please refer to FIG. 1L, which is the cross-sectional schematic showingan embodiment of a method for forming cavity of bulk acoustic waveresonator of the present invention. The embodiment of the bulk acousticwave resonator shown in FIG. 1L is also fabricated from the epitaxialstructure of the embodiment shown in FIG. 1J. The main structure of theembodiment in FIG. 1L is basically the same as that of the embodiment inFIG. 1K, except that in the Step A3, the insulating layer 11 is polishedsuch that the sacrificial epitaxial structure mesa 60 is exposed; thenforming the bulk acoustic wave resonance structure 3 on the polishedsurface 41 and etching the sacrificial epitaxial structure mesa 60 toform the cavity 40 (similar to FIG. 1G and FIG. 1H). Therefore, the bulkacoustic wave resonance structure 3 does not have the frequency tuningstructure 50 as the embodiment shown in FIG. 1K.

Furthermore, please refer to FIGS. 2A˜2F, which are the cross-sectionalschematics showing steps of an embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. As shown in FIG. 2F, the structure of theembodiment comprises at least one first bulk acoustic wave resonator 1and at least one second bulk acoustic wave resonator 1′ formed on asubstrate 10 respectively. In current embodiment, the at least one firstbulk acoustic wave resonator 1 may be a series resonator, while the atleast one second bulk acoustic wave resonator 1′ may be a shuntresonator. The at least one first bulk acoustic wave resonator 1comprises at least one first bulk acoustic wave resonance structure 3, afirst frequency tuning structure 50 and at least one first cavity 40.The at least one second bulk acoustic wave resonator 1′ comprises atleast one second bulk acoustic wave resonance structure 3′, a secondfrequency tuning structure 50′ and at least one second cavity 40′. Amethod of frequency tuning for bulk acoustic wave resonator of bulkacoustic wave filter of the present invention comprises following stepsof: Step B1: (please referring to FIG. 2B) forming a plural ofsacrificial structure mesas on a substrate 10, wherein the plural ofsacrificial structure mesas comprise at least one first sacrificialstructure mesa 6 and at least one second sacrificial structure mesa 6′,wherein a height of the at least one first sacrificial structure mesa 6is greater than a height of the at least one second sacrificialstructure mesa 6′, wherein the at least one first sacrificial structuremesa 6 and the at least one second sacrificial structure mesa 6′ have afirst height difference HD1; in current embodiment, the substrate 10 maybe a semiconductor substrate; the plural of sacrificial structure mesasare made of at least one material selected from the group consisting of:metal, alloy and epitaxial structure; Step B2: (please referring to FIG.2C) forming an insulating layer 11 on the plural of sacrificialstructure mesas and the substrate 10; wherein the insulating layer 11 ismade of at least one material selected from the group consisting of:silicon nitride (SiN_(x)), silicon oxide (SiO₂) and polymer; Step B3:(please referring to FIG. 2D) polishing the insulating layer 11 by achemical-mechanical planarization process to form a polished surface 41;Step B4: (please referring to FIG. 2E) forming a plural of bulk acousticwave resonance structures on the polished surface 41 (In all theembodiments of the bulk acoustic wave filter of the present invention,the plural of bulk acoustic wave resonance structures are all formed onan extending plane 43; in current embodiment, the extending plane 43 iscoincident with the polished surface 41), wherein the plural of bulkacoustic wave resonance structures comprise at least one first bulkacoustic wave resonance structure 3 and at least one second bulkacoustic wave resonance structure 3′, the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′ are located above the at least onefirst sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′ respectively, wherein the Step B4comprises following steps of: Step B41: forming a bottom electrode layer30 on the polished surface 41; Step B42: forming a piezoelectric layer31 on the bottom electrode layer 30; and Step B43: forming a topelectrode layer 32 on the piezoelectric layer 31; and Step B5: (pleasereferring to FIG. 2F) etching the plural of sacrificial structure mesasto form a plural of cavities, wherein the plural of cavities are locatedunder the plural of bulk acoustic wave resonance structuresrespectively; wherein the plural of cavities comprise at least one firstcavity 40 and at least one second cavity 40′, the at least one firstcavity 40 and the at least one second cavity 40′ are located under theat least one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′ respectively;wherein in the Step B3, the insulating layer 11 is polished such thatthe at least one first sacrificial structure mesa 6 and the at least onesecond sacrificial structure mesa 6′ are not exposed, thereby theinsulating layer 11 under the polished surface 41 and located under theat least one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′ form a firstfrequency tuning structure 50 of the at least one first bulk acousticwave resonance structure 3 and a second frequency tuning structure 50′of the at least one second bulk acoustic wave resonance structure 3′respectively, wherein the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ have a first thickness differenceTD1, the first thickness difference TD1 is equal to the first heightdifference HD1; wherein a first resonance frequency F1 of the at leastone first bulk acoustic wave resonance structure 3 will be tuned down bythe first frequency tuning structure 50, while a second resonancefrequency F2 of the at least one second bulk acoustic wave resonancestructure 3′ will be tuned down by the second frequency tuning structure50′. Since a thickness of the second frequency tuning structure 50′ isthicker than a thickness of the first frequency tuning structure 50 suchthat the second resonance frequency F2 of the at least one second bulkacoustic wave resonance structure 3′ is tuned down to be lower than thefirst resonance frequency F1 of the at least one first bulk acousticwave resonance structure 3. Therefore, the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′ have a first resonance frequencydifference FD1, the first resonance frequency difference FD1 iscorrelated to the first thickness difference TD1 of the first frequencytuning structure 50 and the second frequency tuning structure 50′; thatis that the first resonance frequency difference FD1 is correlated tothe first height difference HD1 of the at least one first sacrificialstructure mesa 6 and the at least one second sacrificial structure mesa6′; thereby adjusting the first height difference HD1 is capable oftuning the first resonance frequency difference FD1 of the at least onefirst bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′. Since the dimension ofthe substrate 10 is much greater than the dimension of the bulk acousticwave resonator, when polishing the insulating layer 11 by achemical-mechanical planarization process, usually the polished-offthickness of the insulating layer 11 at the location near the center ofthe substrate 10 is not equal to the polished-off thickness of theinsulating layer 11 at the location away from the center of thesubstrate 10. However, for the adjacent bulk acoustic wave resonators,especially for the plural of bulk acoustic wave resonators of the samebulk acoustic wave filter, the polished-off thickness of the insulatinglayer 11 is almost the same. One of the characteristics of the presentinvention is that the first thickness difference TD1 of the firstfrequency tuning structure 50 and the second frequency tuning structure50′ of the same bulk acoustic wave filter does not vary depending on thelocation near the center of the substrate 10 or the location away fromthe center of the substrate 10. That is to say that the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′ of the same bulk acoustic wave filter does notvary depending on the location near the center of the substrate 10 orthe location away from the center of the substrate 10. Within the samebulk acoustic wave filter, the first resonance frequency difference FD1of the at least one first bulk acoustic wave resonance structure 3 andthe at least one second bulk acoustic wave resonance structure 3′ iscorrelated to the first thickness difference TD1 of the first frequencytuning structure 50 and the second frequency tuning structure 50′,correlated to the first height difference HD1 of the at least one firstsacrificial structure mesa 6 and the at least one second sacrificialstructure mesa 6′, and also correlated to the material of the firstfrequency tuning structure 50 and the second frequency tuning structure50′. By adjusting the first height difference HD1 or choosing differentkinds of materials of the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ is capable of tuning the firstresonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′. Moreover, in the presentinvention, the first resonance frequency difference FD1 of the at leastone first bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′ of the same bulkacoustic wave filter does not vary depending on the location near thecenter of the substrate 10 or the location away from the center of thesubstrate 10. This is one of the main features of the present inventionand is a great help for the later trimming process. Since the firstresonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′ of the same bulk acoustic wavefilter on any area of the wafer may be precisely controlled and does notvary depending on the location, so that the time cost of the trimmingprocess may be significantly reduced. In some embodiments, the substrate10 may be a compound semiconductor substrate; the plural of sacrificialstructure mesas may be made of an epitaxial structure; and wherein theStep B1 comprises following steps of: Step B11: (please referring toFIG. 2A) forming a sacrificial structure 21 on the substrate 10; StepB12: etching the sacrificial structure 21 to form the plural ofsacrificial structure mesas, wherein the plural of sacrificial structuremesas comprise the at least one first sacrificial structure mesa 6(21)and the at least one second sacrificial structure mesa 6′(21), whereinthe plural of sacrificial structure mesas have the same height, and theStep B13: (please referring to FIG. 2B) etching the at least one firstsacrificial structure mesa 6 and the at least one second sacrificialstructure mesa 6′ or etching the at least one second sacrificialstructure mesa 6′ such that the at least one first sacrificial structuremesa 6 and the at least one second sacrificial structure mesa 6′ havethe first height difference HD1.

Please refer to FIGS. 2G and 2H, which are the cross-sectionalschematics showing steps of another embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. In current embodiment, the substrate 10 may be asemiconductor substrate; the plural of sacrificial structure mesas maybe made of at least one material selected from the group consisting of:metal, alloy and epitaxial structure. The main steps for forming theembodiment shown in FIG. 2H are basically the same as those for formingthe embodiment shown in FIG. 2F, except that in the Step B3, theinsulating layer 11 is polished such that the at least one firstsacrificial structure mesa 6 is exposed and the at least one secondsacrificial structure mesa 6′ is not exposed (please referring to FIG.2G), thereby the insulating layer 11 under the polished surface 41 (theextending plane 43) and located under the at least one second bulkacoustic wave resonance structure 3′ forms a second frequency tuningstructure 50′ of the at least one second bulk acoustic wave resonancestructure 3′. As shown in FIG. 2H, the second frequency tuning structure50′ has a thickness T2. The thickness T2 of the second frequency tuningstructure 50′ is equal to the first height difference HD1. In currentembodiment, there is no such a first frequency tuning structure 50 asshown in the embodiment of FIG. 2F. Therefore, the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′ is correlated to the thickness T2 of the secondfrequency tuning structure 50′; that is that the first resonancefrequency difference FD1 is correlated to the first height differenceHD1 of the at least one first sacrificial structure mesa 6 and the atleast one second sacrificial structure mesa 6′; thereby adjusting thefirst height difference HD1 is capable of tuning the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′.

Please refer to FIG. 2I, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.The main structure of the embodiment in FIG. 2I is basically the same asthat of the embodiment in FIG. 2F, except that a bottom etching stoplayer 20 is further formed on the substrate 10, wherein the insulatinglayer 11 is formed on the bottom etching stop layer 20. The at least onefirst cavity 40 and the at least one second cavity 40′ are located abovethe bottom etching stop layer 20. The main steps for forming theembodiment shown in FIG. 2I are basically the same as those for formingthe embodiment shown in FIG. 2F, except that before the Step B11,further comprises a following step of: forming a bottom etching stoplayer 20 on the substrate 10, wherein in the Step B11 the sacrificialstructure 21 is formed on the bottom etching stop layer 20. In the StepB2, the insulating layer 11 is formed on the plural of sacrificialstructure mesas and the bottom etching stop layer 20. In currentembodiment, the substrate 10 may be a compound semiconductor substrate;the plural of sacrificial structure mesas (the sacrificial structure 21)may be made of an epitaxial structure. In some embodiments, thesubstrate 10 is made of GaAs; the sacrificial structure 21 comprises asacrificial epitaxial layer, wherein the sacrificial epitaxial layer ismade of GaAs, the sacrificial epitaxial layer has a thickness between 50nm and 5000 nm; the bottom etching stop layer 20 is made of InGaP,wherein the bottom etching stop layer 20 has a thickness between 20 nmand 500 nm. In some other embodiments, the substrate 10 is made of InP;the sacrificial structure 21 comprises a sacrificial epitaxial layer,wherein the sacrificial epitaxial layer is made of InGaAs, thesacrificial epitaxial layer has a thickness between 50 nm and 5000 nm;the bottom etching stop layer 20 is made of InP, wherein the bottometching stop layer 20 has a thickness between 20 nm and 500 nm.

Please refer to FIG. 2J, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.The substrate 10 may be a compound semiconductor substrate; the pluralof sacrificial structure mesas may be made of an epitaxial structure.The main structure of the embodiment in FIG. 2J is basically the same asthat of the embodiment in FIG. 2I, except that in the Step B3, theinsulating layer 11 is polished such that the at least one firstsacrificial structure mesa 6 is exposed and the at least one secondsacrificial structure mesa 6′ is not exposed (similar to FIG. 2G),thereby the insulating layer 11 under the polished surface 41 (theextending plane 43) and located under the at least one second bulkacoustic wave resonance structure 3′ forms a second frequency tuningstructure 50′ of the at least one second bulk acoustic wave resonancestructure 3′.

Please refer to FIGS. 2K˜2N, which are the cross-sectional schematicsshowing steps of another embodiment of a method of frequency tuning forbulk acoustic wave resonator of bulk acoustic wave filter of the presentinvention. In the embodiment of FIG. 2K, the substrate 10 may be acompound semiconductor substrate; the sacrificial structure 21 may bemade of an epitaxial structure. The main structure of the epitaxialstructure of the embodiment in FIG. 2K is basically the same as that ofthe embodiment in FIG. 2A, except that the sacrificial structure 21comprises a sacrificial epitaxial layer 27, a first etching stop layer22 and a first fine tuning layer 23, wherein the sacrificial epitaxiallayer 27 is formed on the substrate 10, the first etching stop layer 22is formed on the sacrificial epitaxial layer 27, the first fine tuninglayer 23 is formed on the first etching stop layer 22. As shown in FIG.2L, the sacrificial structure 21 is etched to form the plural ofsacrificial structure mesas, wherein the plural of sacrificial structuremesas comprise the at least one first sacrificial structure mesa 6 andthe at least one second sacrificial structure mesa 6′, wherein theplural of sacrificial structure mesas have the same height (the StepB12). As shown in FIG. 2M, the first fine tuning layer 23 has athickness FT1. Etching the first fine tuning layer 23 of the at leastone second sacrificial structure mesa 6′ such that the at least onefirst sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′ have the first height difference HD1 (theStep B13). After the Step B2, the Step B3 and the Step B4, the result isshown in FIG. 2N. After etching (the Step B5) the at least one firstsacrificial structure mesa 6 and the at least one second sacrificialstructure mesa 6′ in FIG. 2N, the result is the embodiment of FIG. 2F.The first height difference HD1 is determined by the thickness FT1 ofthe first fine tuning layer 23, therefore, it helps to precisely adjustthe first height difference HD1. It also helps to precisely tune thefirst resonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′. In some embodiments, the substrate10 is made of GaAs; the sacrificial epitaxial layer 27 is made of GaAs;the first etching stop layer 22 is made of AlAs or InGaP, wherein thefirst etching stop layer 22 has a thickness between 1 nm and 50 nm thefirst fine tuning layer 23 is made of GaAs, wherein the thickness FT1 ofthe first fine tuning layer 23 is between 1 nm and 300 nm. In some otherembodiments, the substrate 10 is made of InP; the sacrificial epitaxiallayer 27 is made of InGaAs; the first etching stop layer 22 is made ofInP, wherein the first etching stop layer 22 has a thickness between 1nm and 50 nm; the first fine tuning layer 23 is made of InGaAs, whereinthe thickness FT1 of the first fine tuning layer 23 is between 1 nm and300 nm.

Please refer to FIGS. 3A˜3G, which are the cross-sectional schematicsshowing steps of an embodiment of a method of frequency tuning for bulkacoustic wave resonator of bulk acoustic wave filter of the presentinvention. Using a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention to format least one first bulk acoustic wave resonator 1 and at least onesecond bulk acoustic wave resonator 1′ (please referring to FIG. 3G)comprises following steps of: Step C1: forming a plural of sacrificialstructure mesas on a substrate 10, wherein the plural of sacrificialstructure mesas have the same height, wherein the plural of sacrificialstructure mesas comprise at least one first sacrificial structure mesa 6and at least one second sacrificial structure mesa 6′; in currentembodiment, the substrate 10 may be a semiconductor substrate; theplural of sacrificial structure mesas may be made of at least onematerial selected from the group consisting of: metal, alloy andepitaxial structure; Step C2: (please referring to FIG. 3A) forming aninsulating layer 11 on the plural of sacrificial structure mesas and thesubstrate 10; Step C3: (please referring to FIG. 3B) polishing theinsulating layer 11 by a prior chemical-mechanical planarization processto form a pre-polished surface 42 such that the plural of sacrificialstructure mesas are exposed; Step C4: (please referring to FIG. 3C)etching the at least one second sacrificial structure mesa 6′ such thatthe at least one first sacrificial structure mesa 6 and the at least onesecond sacrificial structure mesa 6′ have a first height difference HD1,wherein a height of the at least one first sacrificial structure mesa 6is greater than a height of the at least one second sacrificialstructure mesa 6′; Step C5: (please referring to FIG. 3D-FIG. 3F)forming a plural of bulk acoustic wave resonance structures, wherein theplural of bulk acoustic wave resonance structures comprise at least onefirst bulk acoustic wave resonance structure 3 and at least one secondbulk acoustic wave resonance structure 3′, wherein the at least onefirst bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′ are located above theat least one first sacrificial structure mesa 6 and the at least onesecond sacrificial structure mesa 6′ respectively, wherein the Step C5comprises following steps of: Step C51: forming a second polish layer 51on the plural of sacrificial structure mesas and the insulating layer11, wherein the second polish layer 51 is made of insulator, wherein thematerial of the insulator of the second polish layer 51 may be at leastone material selected from the group consisting of: silicon nitride(SiN_(x)), silicon oxide (SiO₂), aluminium nitride (AlN) and zinc oxide(ZnO); Step C52: polishing the second polish layer 51 by achemical-mechanical planarization process to form a polished surface 41such that the at least one first sacrificial structure mesa 6 and the atleast one second sacrificial structure mesa 6′ are not exposed, therebythe second polish layer 51 under the polished surface 41 and locatedunder the at least one first bulk acoustic wave resonance structure 3and the at least one second bulk acoustic wave resonance structure 3′form a first frequency tuning structure 50 of the at least one firstbulk acoustic wave resonance structure 3 and a second frequency tuningstructure 50′ of the at least one second bulk acoustic wave resonancestructure 3′ respectively, wherein the first frequency tuning structure50 and the second frequency tuning structure 50′ have a first thicknessdifference TD1, the first thickness difference TD1 is equal to the firstheight difference HD1; Step C53: forming a bottom electrode layer 30 onthe polished surface 41 (as mentioned above, the plural of bulk acousticwave resonance structures are formed on an extending plane 43; incurrent embodiment, the extending plane 43 is coincident with thepolished surface 41); Step C54: forming a piezoelectric layer 31 on thebottom electrode layer 30; and Step C55: forming a top electrode layer32 on the piezoelectric layer 31; and Step C6: (please referring to FIG.3G) etching the plural of sacrificial structure mesas to form a pluralof cavities, wherein the plural of cavities are located under the pluralof bulk acoustic wave resonance structures respectively; wherein theplural of cavities comprise at least one first cavity 40 and at leastone second cavity 40′; wherein the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′ have a first resonance frequency difference FD1,the first resonance frequency difference FD1 is correlated to the firstthickness difference TD1 of the first frequency tuning structure 50 andthe second frequency tuning structure 50′; that is that the firstresonance frequency difference FD1 is correlated to the first heightdifference HD1, thereby adjusting the first height difference HD1 iscapable of tuning the first resonance frequency difference FD1 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′. In someembodiments, the substrate 10 may be a compound semiconductor substrate;the plural of sacrificial structure mesas may be made of an epitaxialstructure; and wherein the Step C1 comprises following steps of: StepC11: forming a sacrificial structure 21 on the substrate 10; and StepC12: etching the sacrificial structure 21 to form the plural ofsacrificial structure mesas, wherein the plural of sacrificial structuremesas have the same height.

Please refer to FIGS. 3H and 3I, which are the cross-sectionalschematics showing steps of another embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. In current embodiment, the substrate 10 may be asemiconductor substrate; the plural of sacrificial structure mesas maybe made of at least one material selected from the group consisting of:metal, alloy and epitaxial structure. The main steps for forming theembodiment shown in FIG. 3I are basically the same as those for formingthe embodiment shown in FIG. 3G, except that in the Step C52, the secondpolish layer 51 is polished such that the at least one first sacrificialstructure mesa 6 is exposed and the at least one second sacrificialstructure mesa 6′ is not exposed (please referring to FIG. 3H), therebythe second polish layer 51 under the polished surface 41 (the extendingplane 43) and located under the at least one second bulk acoustic waveresonance structure 3′ forms a second frequency tuning structure 50′ ofthe at least one second bulk acoustic wave resonance structure 3′(please referring to FIG. 3I), wherein the second frequency tuningstructure 50′ has a thickness T2, the thickness T2 of the secondfrequency tuning structure 50′ is equal to the first height differenceHD1. In current embodiment, there is no such a first frequency tuningstructure 50 as shown in the embodiment of FIG. 3G. Therefore, the firstresonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′ is correlated to the thickness T2of the second frequency tuning structure 50′; that is that the firstresonance frequency difference FD1 is correlated to the first heightdifference HD1 of the at least one first sacrificial structure mesa 6and the at least one second sacrificial structure mesa 6′, therebyadjusting the first height difference HD1 is capable of tuning the firstresonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′. In current embodiment, the secondpolish layer 51 may be made of at least one material selected from thegroup consisting of: metal, alloy and insulator.

Please refer to FIGS. 3J and 3K, which are the cross-sectionalschematics showing steps of another embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. In current embodiment, the substrate 10 may be asemiconductor substrate; the plural of sacrificial structure mesas maybe made of at least one material selected from the group consisting of:metal, alloy and epitaxial structure. The main steps for forming theembodiment shown in FIG. 3K are basically the same as those for formingthe embodiment shown in FIG. 3G, except that in the Step C5, the pluralof bulk acoustic wave resonance structures are formed on an extendingplane 43, wherein the extending plane 43 is coincident with thepre-polished surface 42, wherein the Step C5 comprises following stepsof: Step C51′: (please referring to FIG. 3D) forming a second polishlayer 51 on the plural of sacrificial structure mesas and the insulatinglayer 11, wherein the second polish layer 51 is made of at least onematerial selected from the group consisting of: metal and alloy; in apreferable embodiment, the second polish layer 51 is made of at leastone material selected from the group consisting of Ru, Ti, Mo, Pt, Au,Al and W; Step C52′: (please referring to FIG. 3E) polishing the secondpolish layer 51 by a chemical-mechanical planarization process to form apolished surface 41 such that the plural of sacrificial structure mesasare not exposed; Step C53′: (please referring to FIG. 3J) patterning thesecond polish layer 51; Step C54′: forming a piezoelectric layer 31 onthe polished surface 41; and Step C55′: forming a top electrode layer 32on the piezoelectric layer 31. After the Step C6 etching the plural ofsacrificial structure mesas, the embodiment of FIG. 3K is formed. In theStep C4, the at least one second sacrificial structure mesa 6′ isetched; wherein the second polish layer 51 above the pre-polishedsurface 42 (the extending plane 43), under the polished surface 41 andlocated under the at least one first bulk acoustic wave resonancestructure 3 forms a bottom electrode layer 30 of the at least one firstbulk acoustic wave resonance structure 3; wherein the second polishlayer 51 above the pre-polished surface 42 (the extending plane 43),under the polished surface 41 and located under the at least one secondbulk acoustic wave resonance structure 3′ forms a bottom electrode layer30 of the at least one second bulk acoustic wave resonance structure 3′;wherein the second polish layer 51 under the pre-polished surface 42(the extending plane 43) and located under the at least one second bulkacoustic wave resonance structure 3′ forms a second frequency tuningstructure 50′ of the at least one second bulk acoustic wave resonancestructure 3′; wherein the second frequency tuning structure 50′ has athickness T2, the thickness T2 of the second frequency tuning structure50′ is equal to the first height difference HD1: thereby adjusting thefirst height difference HD1 is capable of tuning the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′.

Please refer to FIG. 3L, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention. Incurrent embodiment, the substrate 10 may be a semiconductor substrate;the plural of sacrificial structure mesas may be made of at least onematerial selected from the group consisting of: metal, alloy andepitaxial structure. The main steps for forming the embodiment shown inFIG. 3L are basically the same as those for forming the embodiment shownin FIG. 3G, except that the Step C5 comprises following steps of: StepC51″: (please referring to FIG. 3D) forming a second polish layer 51 onthe plural of sacrificial structure mesas and the insulating layer 11,wherein the second polish layer 51 is made of at least one materialselected from the group consisting of: metal, alloy and insulator; StepC52″: (please referring to FIG. 3E) polishing the second polish layer 51by a chemical-mechanical planarization process to form a polishedsurface 41 such that the at least one first sacrificial structure mesa 6and the at least one second sacrificial structure mesa 6′ are notexposed; Step C53″: (please referring to FIG. 3J) patterning the secondpolish layer 51; Step C54″: forming a bottom electrode layer 30 on thepolished surface 41 (the extending plane 43); Step C55″: forming apiezoelectric layer 31 on the bottom electrode layer 30; and Step C56″:forming a top electrode layer 32 on the piezoelectric layer 31. Theembodiment of FIG. 3L is formed after the Step C6; thereby the secondpolish layer 51 under the polished surface 41 and located under the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′ form a firstfrequency tuning structure 50 of the at least one first bulk acousticwave resonance structure 3 and a second frequency tuning structure 50′of the at least one second bulk acoustic wave resonance structure 3′respectively, wherein the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ have a first thickness differenceTD1, the first thickness difference TD1 is equal to the first heightdifference HD1; thereby adjusting the first height difference HD1 iscapable of tuning the first resonance frequency difference FD1 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′.

In the embodiments of FIGS. 3G and 3I, in the Step C2 (please referringto FIG. 3A), a very thick insulating layer 11 is formed firstly, whereina thickness of the insulating layer 11 must be higher than a height ofthe plural of sacrificial structure mesas. In the Step C3 (pleasereferring to FIG. 3B), a chemical-mechanical planarization process mustpolish the insulating layer 11 a polished-off thickness which must be atleast equal to or greater than the height of the plural of sacrificialstructure mesas. However, there is a drawback to the chemical-mechanicalplanarization process. That is that when the required polished-offthickness is too thick, the uniformity of the polished surface will bedeteriorated. In current embodiment, since the required polished-offthickness of the insulating layer 11 is very thick such that theuniformity of the pre-polished surface 42 is deteriorated afterpolished. However, the required polished-off thickness of the secondpolish layer 51 formed in the later Step C51 is very thin compared tothe required polished-off thickness of the insulating layer 11. Thepolished-off thickness of the second polish layer 51 is only required tobe greater than the first height difference HD1. Therefore, theuniformity of the polished surface 41 formed after polishing the secondpolish layer 51 by the chemical-mechanical planarization process in theStep C52 is not deteriorated. Hence, to form the bottom electrode layer30 of the at least one first bulk acoustic wave resonator 1 and the atleast one second bulk acoustic wave resonator 1′ on the polished surface41 helps to enhance the resonance characteristics of the at least onefirst bulk acoustic wave resonator 1 and the at least one second bulkacoustic wave resonator 1′. Similarly, it's also the same in theembodiment of FIG. 3L. While in the embodiment of FIG. 3K, to form thepiezoelectric layer 31 of the at least one first bulk acoustic waveresonator 1 and the at least one second bulk acoustic wave resonator 1′on the polished surface 41 also helps to enhance the resonancecharacteristics of the at least one first bulk acoustic wave resonator 1and the at least one second bulk acoustic wave resonator 1′.

The embodiments of FIGS. 3G, 3I, 3K and 3L may be formed from thesacrificial structure similar to that of FIG. 2K, wherein the substrate10 is a compound semiconductor substrate; wherein the sacrificialstructure 21 comprises a sacrificial epitaxial layer 27, a first etchingstop layer 22 and a first fine tuning layer 23, wherein the sacrificialepitaxial layer 27 is formed on the substrate 10, the first etching stoplayer 22 is formed on the sacrificial epitaxial layer 27, the first finetuning layer 23 is formed on the first etching stop layer 22, whereinthe first fine tuning layer 23 has a thickness FT1. In the Step C4, thefirst fine tuning layer 23 of the at least one second sacrificialstructure mesa 6′ is etched such that the at least one first sacrificialstructure mesa 6 and the at least one second sacrificial structure mesa6′ have the first height difference HD1, thereby the first heightdifference HD1 is determined by the thickness FT1 of the first finetuning layer 23. Therefore, it helps to precisely adjust the firstheight difference HD1. It also helps to precisely tune the firstresonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′.

Please refer to FIGS. 4A˜4D, which are the cross-sectional schematicsshowing steps of an embodiment of a method of frequency tuning for bulkacoustic wave resonator of bulk acoustic wave filter of the presentinvention. The main steps for forming the embodiment shown in FIG. 4Dare basically the same as those for forming the embodiment shown in FIG.3G, except that in the Step C4 (please referring to FIG. 4A), the atleast one first sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′ are etched such that the at least onefirst sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′ have a first height difference HD1,wherein a height of the at least one first sacrificial structure mesa 6is greater than a height of the at least one second sacrificialstructure mesa 6′. The embodiment of FIG. 4D is formed after the StepC51 (please referring to FIG. 4B), the Step C52 (please referring toFIG. 4C), the Step C53˜the Step C55 and the Step C6, wherein the secondpolish layer 51 is made of insulator. In current embodiment, thesubstrate 10 may be a semiconductor substrate; the plural of sacrificialstructure mesas may be made of at least one material selected from thegroup consisting of: metal, alloy and epitaxial structure.

Please refer to FIGS. 4E and 4F, which are the cross-sectionalschematics showing steps of another embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. In current embodiment, the substrate 10 may be asemiconductor substrate; the plural of sacrificial structure mesas maybe made of at least one material selected from the group consisting of:metal, alloy and epitaxial structure. The main steps for forming theembodiment shown in FIG. 4F are basically the same as those for formingthe embodiment shown in FIG. 4D, except that in the Step C52, (pleasealso referring to FIG. 4E) the second polish layer 51 is polished suchthat the polished surface 41 (the extending plane 43) is coincident withthe pre-polished surface 42 or such that the polished surface 41 islower than the pre-polished surface 42, and wherein the at least onefirst sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′ are not exposed. In the embodiment of FIG.4F, the second polish layer 51 may be made of at least one materialselected from the group consisting of: metal, alloy and insulator.

Please refer to FIGS. 4G and 4H, which are the cross-sectionalschematics showing steps of another embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. In current embodiment, the substrate 10 may be asemiconductor substrate; the plural of sacrificial structure mesas maybe made of at least one material selected from the group consisting of:metal, alloy and epitaxial structure. The main steps for forming theembodiment shown in FIG. 4H are basically the same as those for formingthe embodiment shown in FIG. 3K, except that in the Step C4: (pleasereferring to FIG. 4A) the at least one first sacrificial structure mesa6 and the at least one second sacrificial structure mesa 6′ are etchedsuch that the at least one first sacrificial structure mesa 6 and the atleast one second sacrificial structure mesa 6′ have a first heightdifference HD1, wherein a height of the at least one first sacrificialstructure mesa 6 is greater than a height of the at least one secondsacrificial structure mesa 6′. In the Step C5, the plural of bulkacoustic wave resonance structures are formed on an extending plane 43,wherein the extending plane 43 is coincident with the pre-polishedsurface 42. After the Step C53′: (please referring to FIG. 4G)patterning the second polish layer 51, and the Step C54′, the Step C55′and the Step C6 (please referring to FIG. 4H), the second polish layer51 above the pre-polished surface 42 (the extending plane 43), under thepolished surface 41 and located under the at least one first bulkacoustic wave resonance structure 3 forms a bottom electrode layer 30 ofthe at least one first bulk acoustic wave resonance structure 3; whereinthe second polish layer 51 under the pre-polished surface 42 (theextending plane 43) and located under the at least one first bulkacoustic wave resonance structure 3 forms a first frequency tuningstructure 50 of the at least one first bulk acoustic wave resonancestructure 3; wherein the second polish layer 51 above the pre-polishedsurface 42 (the extending plane 43), under the polished surface 41 andlocated under the at least one second bulk acoustic wave resonancestructure 3′ forms a bottom electrode layer 30 of the at least onesecond bulk acoustic wave resonance structure 3′; wherein the secondpolish layer 51 under the pre-polished surface 42 (the extending plane43) and located under the at least one second bulk acoustic waveresonance structure 3′ forms a second frequency tuning structure 50′ ofthe at least one second bulk acoustic wave resonance structure 3′;wherein the first frequency tuning structure 50 and the second frequencytuning structure 50′ have a first thickness difference TD1, the firstthickness difference TD1 is equal to the first height difference HD1,thereby adjusting the first height difference HD1 is capable of tuningthe first resonance frequency difference FD1 of the at least one firstbulk acoustic wave resonance structure 3 and the at least one secondbulk acoustic wave resonance structure 3′. The second polish layer 51may be made of at least one material selected from the group consistingof: metal and alloy; wherein in a preferable embodiment, the secondpolish layer 51 is made of at least one material selected from the groupconsisting of: Ru, Ti, Mo, Pt, Au, Al and W.

Please refer to FIG. 4I, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention. Incurrent embodiment, the substrate 10 may be a semiconductor substrate;the plural of sacrificial structure mesas may be made of at least onematerial selected from the group consisting of: metal, alloy andepitaxial structure. The main steps for forming the embodiment shown inFIG. 4I are basically the same as those for forming the embodiment shownin FIG. 3L, except that in the Step C4: (please referring to FIG. 4A)the at least one first sacrificial structure mesa 6 and the at least onesecond sacrificial structure mesa 6′ are etched such that the at leastone first sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′ have a first height difference HD1,wherein a height of the at least one first sacrificial structure mesa 6is greater than a height of the at least one second sacrificialstructure mesa 6′, and wherein in the Step C52″, the second polish layer51 is polished such that the at least one first sacrificial structuremesa 6 and the at least one second sacrificial structure mesa 6′ are notexposed. After the Step C53″: (please referring to FIG. 4G) patterningthe second polish layer 51, and the Step C54″˜the Step C56″ and the StepC6 (please referring to FIG. 4I), the second polish layer 51 under thepolished surface 41 (the extending plane 43) and located under the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′ form a firstfrequency tuning structure 50 of the at least one first bulk acousticwave resonance structure 3 and a second frequency tuning structure 50′of the at least one second bulk acoustic wave resonance structure 3′respectively, wherein the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ have a first thickness differenceTD1, the first thickness difference TD1 is equal to the first heightdifference HD1; thereby adjusting the first height difference HD1 iscapable of tuning the first resonance frequency difference FD1 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′. In currentembodiment, the second polish layer 51 may be made of at least onematerial selected from the group consisting of: metal, alloy andinsulator.

Please refer to FIGS. 4J˜4M, which are the cross-sectional schematicsshowing steps of another embodiment of a method of frequency tuning forbulk acoustic wave resonator of bulk acoustic wave filter of the presentinvention. In the embodiment of FIG. 4J, the substrate 10 may be acompound semiconductor substrate; the sacrificial structure 21 may bemade of an epitaxial structure. The main structure of the epitaxialstructure of the embodiment in FIG. 4J is basically the same as that ofthe embodiment in FIG. 2L, except that the sacrificial structure 21comprises a sacrificial epitaxial layer 27, a first etching stop layer22, a first fine tuning layer 23 and a top etching stop layer 26,wherein the Step C1 comprises following steps of: Step C11: forming asacrificial structure 21 on the substrate 10; and Step C12: etching thesacrificial structure 21 to form the plural of sacrificial structuremesas, wherein the plural of sacrificial structure mesas have the sameheight, wherein the plural of sacrificial structure mesas comprise atleast one first sacrificial structure mesa 6 and at least one secondsacrificial structure mesa 6′, wherein the sacrificial epitaxial layer27 is formed on the substrate 10, the first etching stop layer 22 isformed on the sacrificial epitaxial layer 27, the first fine tuninglayer 23 is formed on the first etching stop layer 22, the top etchingstop layer 26 is formed on the first fine tuning layer 23. The structureof the embodiment of FIG. 4K is formed after the Step C2 and the StepC3. The Step C4 comprises following steps of: Step C41: (pleasereferring to FIG. 4L) etching the top etching stop layer 26 of the atleast one first sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′; and Step C42: (please referring to FIG.4M) etching the first fine tuning layer 23 of the at least one secondsacrificial structure mesa 6′ such that the at least one firstsacrificial structure mesa 6 and the at least one second sacrificialstructure mesa 6′ have the first height difference HD1, wherein thefirst fine tuning layer 23 has a thickness FT1, thereby the first heightdifference HD1 is determined by the thickness FT1 of the first finetuning layer 23, therefore, it helps to precisely adjust the firstheight difference HD1. Hence it helps to precisely adjust the firstthickness difference TD1 of the first frequency tuning structure 50 andthe second frequency tuning structure 50′. It also helps to preciselytune the first resonance frequency difference FD1 of the at least onefirst bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′. The embodiments ofFIGS. 4D, 4F, 4H and 4I may be formed from the structure of FIG. 4M. Toform the embodiments of FIGS. 4D, 4F, 4H and 4I from the epitaxialstructure of FIG. 4M, in the Step C3, the insulating layer 11 ispolished such that the plural of sacrificial structure mesas areexposed. However, during polishing, the plural of sacrificial structuremesas located near the center of the substrate 10 and the plural ofsacrificial structure mesas located away from the center of thesubstrate 10 usually cannot be polished to expose at the same time. Forexample, if the plural of sacrificial structure mesas located away fromthe center of the substrate 10 are exposed earlier, then must continuethe polish process in order to let the plural of sacrificial structuremesas located near the center of the substrate 10 exposed. Therefore,the plural of sacrificial structure mesas located near the center of thesubstrate 10 are over polished. Hence, after polish, the thickness ofthe first fine tuning layer 23 of the plural of sacrificial structuremesas located away from the center of the substrate 10 becomes thinnerthan the thickness of the first fine tuning layer 23 of the plural ofsacrificial structure mesas located near the center of the substrate 10.In order to prevent from that the thickness of the first fine tuninglayer 23 of the plural of sacrificial structure mesas located near thecenter of the substrate 10 is unequal to the thickness of the first finetuning layer 23 of the plural of sacrificial structure mesas locatedaway from the center of the substrate 10, the top etching stop layer 26is introduced such that the thickness of the first fine tuning layer 23of the plural of sacrificial structure mesas located near the center ofthe substrate 10 is capable to be preserved to be equal to the thicknessof the first fine tuning layer 23 of the plural of sacrificial structuremesas located away from the center of the substrate 10. In someembodiments, the substrate 10 is made of GaAs; the sacrificial epitaxiallayer 27 is made of GaAs; the first etching stop layer 22 is made ofAlAs or InGaP, wherein the first etching stop layer 22 has a thicknessbetween 1 nm and 50 nm; the first fine tuning layer 23 is made of GaAs,wherein the thickness FT1 of the first fine tuning layer 23 is between 1nm and 300 nm; the top etching stop layer 26 is made of InGaP, whereinthe top etching stop layer 26 has a thickness between 50 nm and 300 nm.In some other embodiments, the substrate 10 is made of InP; thesacrificial epitaxial layer 27 is made of InGaAs; the first etching stoplayer 22 is made of InP, wherein the first etching stop layer 22 has athickness between 1 nm and 50 nm; the first fine tuning layer 23 is madeof InGaAs, wherein the thickness FT1 of the first fine tuning layer 23is between 1 nm and 300 nm; the top etching stop layer 26 is made ofInP, wherein the top etching stop layer 26 has a thickness between 50 nmand 300 nm.

All the embodiments of FIGS. 2F, 2H, 2I, 2J, 3G, 3I, 3K, 3L, 4D, 4F, 4Hand 4I have a common characteristic, wherein the at least one first bulkacoustic wave resonator 1 and the at least one second bulk acoustic waveresonator 1′ are formed by a method of frequency tuning for bulkacoustic wave resonator of bulk acoustic wave filter of the presentinvention. The common characteristic is that the bottom electrode layer30 of any of the bulk acoustic wave resonance structure (the at leastone first bulk acoustic wave resonance structure 3 or the at least onesecond bulk acoustic wave resonance structure 3′) is formed on anextending plane 43. The common structures of these embodiments comprise:an insulating layer 11 formed on a substrate 10, wherein the insulatinglayer 11 has a plural of cavities; a plural of bulk acoustic waveresonance structures, wherein the plural of bulk acoustic wave resonancestructures are located above the plural of cavities respectively,wherein the plural of bulk acoustic wave resonance structures compriseat least one first bulk acoustic wave resonance structure 3 and at leastone second bulk acoustic wave resonance structure 3′, the plural ofcavities comprise at least one first cavity 40 and at least one secondcavity 40′, the at least one first bulk acoustic wave resonancestructure 3 and the at least one second bulk acoustic wave resonancestructure 3′ are located above the at least one first cavity 40 and theat least one second cavity 40′ respectively, wherein the at least onefirst bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′ have a first resonancefrequency difference FD1; wherein each of the plural of bulk acousticwave resonance structures comprises: a bottom electrode layer 30 formedon an extending plane 43; a piezoelectric layer 31 formed on the bottomelectrode layer 30; and a top electrode layer 32 formed on thepiezoelectric layer 31; and a structure for tuning frequency. While thedifferences between these embodiments are as follows: (1) in theembodiments of FIGS. 2H, 2J, 3I and 3K, the structure for tuningfrequency comprises a structure A: the insulating layer 11 has apolished top surface, the extending plane 43 is coincident with the topsurface of the insulating layer 11, wherein the at least one second bulkacoustic wave resonance structure 3′ comprises a second frequency tuningstructure 50′, the second frequency tuning structure 50′ is formed underthe extending plane 43 and located between the bottom electrode layer 30of the at least one second bulk acoustic wave resonance structure 3′ andthe at least one second cavity 40′, wherein the second frequency tuningstructure 50′ has a thickness T2, the thickness T2 is correlated to thefirst resonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′; (2) in the embodiments of FIGS.2F, 2I, 4F and 4H, the structure for tuning frequency comprises astructure B: the insulating layer 11 has a polished top surface, theextending plane 43 is coincident with the top surface of the insulatinglayer 1, wherein the at least one first bulk acoustic wave resonancestructure 3 and the at least one second bulk acoustic wave resonancestructure 3′ have a first frequency tuning structure 50 and a secondfrequency tuning structure 50′ respectively, wherein the first frequencytuning structure 50 is formed under the extending plane 43 and locatedbetween the bottom electrode layer 30 of the at least one first bulkacoustic wave resonance structure 3 and the at least one first cavity40, the second frequency tuning structure 50′ is formed under theextending plane 43 and located between the bottom electrode layer 30 ofthe at least one second bulk acoustic wave resonance structure 3′ andthe at least one second cavity 40′, wherein the first frequency tuningstructure 50 and the second frequency tuning structure 50′ have a firstthickness difference TD1, the first thickness difference TD1 iscorrelated to a first resonance frequency difference FD1 of the at leastone first bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′; (3) in the embodimentsof FIGS. 3G, 3L, 4D and 4I, the structure for tuning frequency comprisesa structure C: a second polish layer 51 is formed on the insulatinglayer 11 and above the plural of cavities, wherein the second polishlayer 51 has a polished top surface, the extending plane 43 iscoincident with the top surface of the second polish layer 51, whereinthe second polish layer 51 under the extending plane 43 and locatedbetween the bottom electrode layer 30 of the at least one first bulkacoustic wave resonance structure 3 and the at least one first cavity 40forms a first frequency tuning structure 50 of the at least one firstbulk acoustic wave resonance structure 3, wherein the second polishlayer 51 under the extending plane 43 and located between the bottomelectrode layer 30 of the at least one second bulk acoustic waveresonance structure 3′ and the at least one second cavity 40′ forms asecond frequency tuning structure 50′ of the at least one second bulkacoustic wave resonance structure 3′, wherein the first frequency tuningstructure 50 and the second frequency tuning structure 50′ have a firstthickness difference TD1, the first thickness difference TD1 iscorrelated to a first resonance frequency difference FD1 of the at leastone first bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′. In the embodiments ofFIGS. 2F, 2I, 3G, 3L, 4D, 4F, 4H and 4I of the present invention, thecommon characteristic is that: the bottom electrode layer 30 of the atleast one first bulk acoustic wave resonance structure 3 and the bottomelectrode layer 30 of the at least one second bulk acoustic waveresonance structure 3′ are formed on the extending plane 43; and whereinthe first frequency tuning structure 50 and the second frequency tuningstructure 50′ are formed under the extending plane 43. In theembodiments of FIGS. 2H, 2J, 3I and 3K of the present invention, thecommon characteristic is that: the bottom electrode layer 30 of the atleast one first bulk acoustic wave resonance structure 3 and the bottomelectrode layer 30 of the at least one second bulk acoustic waveresonance structure 3′ are formed on the extending plane 43; and whereinthe second frequency tuning structure 50′ is formed under the extendingplane 43.

Please refer to FIGS. 5A˜5C, which are the cross-sectional schematicsshowing steps of an embodiment of a method of frequency tuning for bulkacoustic wave resonator of bulk acoustic wave filter of the presentinvention. In current embodiment, the substrate 10 may be asemiconductor substrate; the plural of sacrificial structure mesas maybe made of at least one material selected from the group consisting of:metal, alloy and epitaxial structure. The main steps for forming theembodiment shown in FIG. 5C are basically the same as those for formingthe embodiment shown in FIG. 2E except that at least one first bulkacoustic wave resonator 1, at least one second bulk acoustic waveresonator 1′ and at least one third bulk acoustic wave resonator 1″ areformed on the substrate 10 respectively. In the Step B1, (pleasereferring to FIG. 5B) the plural of sacrificial structure mesas compriseat least one first sacrificial structure mesa 6, at least one secondsacrificial structure mesa 6′ and at least one third sacrificialstructure mesa 6″; wherein a height of the at least one firstsacrificial structure mesa 6 is greater than a height of the at leastone second sacrificial structure mesa 6′, wherein the at least one firstsacrificial structure mesa 6 and the at least one second sacrificialstructure mesa 6′ have a first height difference HD1; wherein the heightof the at least one first sacrificial structure mesa 6 is greater than aheight of the at least one third sacrificial structure mesa 6″, whereinthe at least one first sacrificial structure mesa 6 and the at least onethird sacrificial structure mesa 6″ have a second height difference HD2.In the Step B4, a plural of bulk acoustic wave resonance structures areformed on the polished surface 41 (the extending plane 43), wherein theplural of bulk acoustic wave resonance structures comprise at least onefirst bulk acoustic wave resonance structure 3, at least one second bulkacoustic wave resonance structure 3′ and at least one third bulkacoustic wave resonance structure 3″, wherein the at least one firstbulk acoustic wave resonance structure 3, the at least one second bulkacoustic wave resonance structure 3′ and the at least one third bulkacoustic wave resonance structure 3″ are located above the at least onefirst sacrificial structure mesa 6, the at least one second sacrificialstructure mesa 6′ and the at least one third sacrificial structure mesa6″ respectively. In the Step B5, the plural of sacrificial structuremesas are etched to form a plural of cavities, wherein the plural ofcavities comprise at least one first cavity 40, at least one secondcavity 40′ and at least one third cavity 40″, the at least one firstcavity 40, the at least one second cavity 40′ and the at least one thirdcavity 40″ are located under the at least one first bulk acoustic waveresonance structure 3, the at least one second bulk acoustic waveresonance structure 3′ and the at least one third bulk acoustic waveresonance structure 3″ respectively. In the Step B3, the insulatinglayer 11 is polished such that the at least one first sacrificialstructure mesa 6, the at least one second sacrificial structure mesa 6′and the at least one third sacrificial structure mesa 6″ are notexposed, thereby the insulating layer 11 under the polished surface 41and located under the at least one first bulk acoustic wave resonancestructure 3, the at least one second bulk acoustic wave resonancestructure 3′ and the at least one third bulk acoustic wave resonancestructure 3″ form a first frequency tuning structure 50 of the at leastone first bulk acoustic wave resonance structure 3, a second frequencytuning structure 50′ of the at least one second bulk acoustic waveresonance structure 3′ and a third frequency tuning structure 50″ of theat least one third bulk acoustic wave resonance structure 3″respectively; wherein the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ have a first thickness differenceTD1, the first thickness difference TD1 is equal to the first heightdifference HD1; wherein the first frequency tuning structure 50 and thethird frequency tuning structure 50″ have a second thickness differenceTD2, the second thickness difference TD2 is equal to the second heightdifference HD2: thereby adjusting the first height difference HD1 iscapable of tuning a first resonance frequency difference FD1 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′; whileadjusting the second height difference HD2 is capable of tuning a secondresonance frequency difference FD2 of the at least one first bulkacoustic wave resonance structure 3 and the at least one third bulkacoustic wave resonance structure 3″. In some embodiments, the substrate10 may be a compound semiconductor substrate; the plural of sacrificialstructure mesas may be made of an epitaxial structure, and wherein theStep B1 comprises following steps of: Step B11: (please referring toFIG. 5A) forming a sacrificial structure 21 on the substrate 10; StepB12: etching the sacrificial structure 21 to form the plural ofsacrificial structure mesas, wherein the plural of sacrificial structuremesas comprise the at least one first sacrificial structure mesa 6(21),the at least one second sacrificial structure mesa 6′(21) and the atleast one third sacrificial structure mesa 6″(21), wherein the plural ofsacrificial structure mesas have the same height, and the Step B13:(please referring to FIG. 5B) etching the at least one first sacrificialstructure mesa 6, the at least one second sacrificial structure mesa 6′and the at least one third sacrificial structure mesa 6″ or etching theat least one second sacrificial structure mesa 6′ and the at least onethird sacrificial structure mesa 6″ such that the at least one firstsacrificial structure mesa 6 and the at least one second sacrificialstructure mesa 6′ have the first height difference HD1 and such that theat least one first sacrificial structure mesa 6 and the at least onethird sacrificial structure mesa 6″ have the second height differenceHD2.

Please refer to FIG. 5D, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention. Incurrent embodiment, the substrate 10 may be a semiconductor substrate;the plural of sacrificial structure mesas may be made of at least onematerial selected from the group consisting of: metal, alloy andepitaxial structure. The main steps for forming the embodiment shown inFIG. 5D are basically the same as those for forming the embodiment shownin FIG. 5C, except that in the Step B3, the insulating layer 11 ispolished such that the at least one first sacrificial structure mesa 6is exposed and the at least one second sacrificial structure mesa 6′ andthe at least one third sacrificial structure mesa 6″ are not exposed,thereby the insulating layer 11 under the polished surface 41 (theextending plane 43) and located under the at least one second bulkacoustic wave resonance structure 3′ forms a second frequency tuningstructure 50′ of the at least one second bulk acoustic wave resonancestructure 3′; and the insulating layer 11 under the polished surface 41(the extending plane 43) and located under the at least one third bulkacoustic wave resonance structure 3″ forms a third frequency tuningstructure 50″ of the at least one third bulk acoustic wave resonancestructure 3″; wherein the second frequency tuning structure 50′ has athickness T2, the thickness T2 of the second frequency tuning structure50′ is equal to the first height difference HD1; wherein the thirdfrequency tuning structure 50″ has a thickness T3, the thickness T3 ofthe third frequency tuning structure 50″ is equal to the second heightdifference HD2. In current embodiment, there is no such a firstfrequency tuning structure 50 as shown in the embodiment of FIG. 5C.Therefore, the first resonance frequency difference FD1 of the at leastone first bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′ is correlated to thethickness T2 of the second frequency tuning structure 50′; that is thatthe first resonance frequency difference FD1 is correlated to the firstheight difference HD1 of the at least one first sacrificial structuremesa 6 and the at least one second sacrificial structure mesa 6′; thesecond resonance frequency difference FD2 of the at least one first bulkacoustic wave resonance structure 3 and the at least one third bulkacoustic wave resonance structure 3″ is correlated to the thickness T3of the third frequency tuning structure 50″; that is that the secondresonance frequency difference FD2 is correlated to the second heightdifference HD2 of the at least one first sacrificial structure mesa 6and the at least one third sacrificial structure mesa 6″; therebyadjusting the first height difference HD1 is capable of tuning the firstresonance frequency difference FD1 of the at least one first bulkacoustic wave resonance structure 3 and the at least one second bulkacoustic wave resonance structure 3′; thereby adjusting the secondheight difference HD2 is capable of tuning a second resonance frequencydifference FD2 of the at least one first bulk acoustic wave resonancestructure 3 and the at least one third bulk acoustic wave resonancestructure 3″.

Please refer to FIGS. 5E˜5G, which are the cross-sectional schematicsshowing steps of another embodiment of a method of frequency tuning forbulk acoustic wave resonator of bulk acoustic wave filter of the presentinvention. In the embodiment of FIG. 5E, the substrate 10 is a compoundsemiconductor substrate; the sacrificial structure 21 is made of anepitaxial structure. The main epitaxial structures of the embodiments inFIG. 5E˜5G are basically the same as those of the embodiments in FIG.5A˜5B, except that the sacrificial structure 21 comprises a sacrificialepitaxial layer 27, a second etching stop layer 24, a second fine tuninglayer 25, a first etching stop layer 22 and a first fine tuning layer23, wherein the sacrificial epitaxial layer 27 is formed on thesubstrate 10, the second etching stop layer 24 is formed on thesacrificial epitaxial layer 27, the second fine tuning layer 25 isformed on the second etching stop layer 24, the first etching stop layer22 is formed on the second fine tuning layer 25, the first fine tuninglayer 23 is formed on the first etching stop layer 22. As shown in FIG.5F, the sacrificial structure 21 is etched to form the plural ofsacrificial structure mesas such that the plural of sacrificialstructure mesas have the same height, wherein the plural of sacrificialstructure mesas comprise at least one first sacrificial structure mesa6, at least one second sacrificial structure mesa 6′ and at least onethird sacrificial structure mesa 6″. As shown in FIG. 5G, the first finetuning layer 23 has a thickness FT1, the first etching stop layer 22 hasa thickness ET1, the second fine tuning layer 25 has a thickness FT2.The first fine tuning layer 23 of the at least one second sacrificialstructure mesa 6′ is etched such that the at least one first sacrificialstructure mesa 6 and the at least one second sacrificial structure mesa6′ have a first height difference HD1 the first fine tuning layer 23,the first etching stop layer 22 and the second fine tuning layer 25 ofthe at least one third sacrificial structure mesa 6″ are etched suchthat the at least one first sacrificial structure mesa 6 and the atleast one third sacrificial structure mesa 6″ have a second heightdifference HD2. The embodiment of FIG. 5C may be formed from thestructure of FIG. 5G, wherein the first height difference HD1 isdetermined by the thickness FT1 of the first fine tuning layer 23,therefore, it helps to precisely adjust the first height difference HD1.Hence it helps to precisely adjust the first thickness difference TD1 ofthe first frequency tuning structure 50 and the second frequency tuningstructure 50′. It also helps to precisely tune the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′; wherein the second height difference HD2 isdetermined by the thickness FT1 of the first fine tuning layer 23, thethickness ET1 of the first etching stop layer 22 and the thickness FT2of the second fine tuning layer 25, therefore, it helps to preciselyadjust the second height difference HD2. It also helps to precisely tunethe second resonance frequency difference FD2 of the at least one firstbulk acoustic wave resonance structure 3 and the at least one third bulkacoustic wave resonance structure 3″. The embodiment of FIG. 5D may alsobe formed from the structure of FIG. 5G, wherein the first heightdifference HD1 is determined by the thickness FT1 of the first finetuning layer 23, therefore, it helps to precisely adjust the firstheight difference HD1. Hence it helps to precisely adjust the thicknessT2 of the second frequency tuning structure 50′. It also helps toprecisely tune the first resonance frequency difference FD1 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′; wherein thesecond height difference HD2 is determined by the thickness FT1 of thefirst fine tuning layer 23, the thickness ET1 of the first etching stoplayer 22 and the thickness FT2 of the second fine tuning layer 25,therefore, it helps to precisely adjust the second height differenceHD2. Hence it helps to precisely adjust the thickness T3 of the thirdfrequency tuning structure 50″. It also helps to precisely tune thesecond resonance frequency difference FD2 of the at least one first bulkacoustic wave resonance structure 3 and the at least one third bulkacoustic wave resonance structure 3″. In some embodiments, the substrate10 is made of GaAs; the sacrificial epitaxial layer 27 is made of GaAs;the first etching stop layer 22 is made of AlAs or InGaP, wherein thethickness ET1 of the first etching stop layer 22 is between 1 nm and 50nm; the first fine tuning layer 23 is made of GaAs, wherein thethickness FT1 of the first fine tuning layer 23 is between 1 nm and 300nm; the second etching stop layer 24 is made of AlAs or InGaP, whereinthe second etching stop layer 24 has a thickness between 1 nm and 50 nm;the second fine tuning layer 25 is made of GaAs, the thickness FT2 ofthe second fine tuning layer 25 is between 1 nm and 300 nm. In someother embodiments, the substrate 10 is made of InP; the sacrificialepitaxial layer 27 is made of InGaAs; the first etching stop layer 22 ismade of InP, wherein the thickness ET1 of the first etching stop layer22 is between 1 nm and 50 nm; the first fine tuning layer 23 is made ofInGaAs, wherein the thickness FT1 of the first fine tuning layer 23 isbetween 1 nm and 300 nm; the second etching stop layer 24 is made ofInP, wherein the second etching stop layer 24 has a thickness between 1nm and 50 nm; the second fine tuning layer 25 is made of InGaAs, thethickness FT2 of the second fine tuning layer 25 is between 1 nm and 300nm.

Please refer to FIGS. 5H˜5K, which are the cross-sectional schematicsshowing steps of another embodiment of a method of frequency tuning forbulk acoustic wave resonator of bulk acoustic wave filter of the presentinvention. In current embodiment, the substrate 10 is a compoundsemiconductor substrate; the sacrificial structure 21 is made of anepitaxial structure. The embodiment of FIG. 5K is formed from theepitaxial structure of FIG. 5E. The main steps for forming theembodiment shown in FIG. 5K are basically the same as those for formingthe embodiment shown in FIG. 3G, except that at least one first bulkacoustic wave resonator 1, at least one second bulk acoustic waveresonator 1′ and at least one third bulk acoustic wave resonator 1″ areformed on the substrate 10 respectively; wherein the Step C1 comprisesfollowing steps of: Step C11: (please referring to FIG. 5E) forming asacrificial structure 21 on the substrate 10, wherein the sacrificialstructure 21 comprises a sacrificial epitaxial layer 27, a secondetching stop layer 24, a second fine tuning layer 25, a first etchingstop layer 22 and a first fine tuning layer 23, wherein the sacrificialepitaxial layer 27 is formed on the substrate 10, the second etchingstop layer 24 is formed on the sacrificial epitaxial layer 27, thesecond fine tuning layer 25 is formed on the second etching stop layer24, the first etching stop layer 22 is formed on the second fine tuninglayer 25, the first fine tuning layer 23 is formed on the first etchingstop layer 22; and Step C12: (please referring to FIG. 5F) etching thesacrificial structure 21 to form the plural of sacrificial structuremesas, wherein the plural of sacrificial structure mesas have the sameheight, wherein the plural of sacrificial structure mesas comprise atleast one first sacrificial structure mesa 6, at least one secondsacrificial structure mesa 6′ and at least one third sacrificialstructure mesa 6″. After the Step C2 and the Step C3, the structure ofFIG. 5H is formed. In the Step C4, (please referring to FIG. 5I) thefirst fine tuning layer 23 of the at least one second sacrificialstructure mesa 6′ is etched such that the at least one first sacrificialstructure mesa 6 and the at least one second sacrificial structure mesa6′ have a first height difference HD1; and the first fine tuning layer23, the first etching stop layer 22 and the second fine tuning layer 25of the at least one third sacrificial structure mesa 6″ are etched suchthat the at least one first sacrificial structure mesa 6 and the atleast one third sacrificial structure mesa 6″ have a second heightdifference HD2, wherein the first fine tuning layer 23 has a thicknessFT1, the first etching stop layer 22 has a thickness ET1, the secondfine tuning layer 25 has a thickness FT2. In the Step C5, the plural ofbulk acoustic wave resonance structures are formed, wherein the pluralof bulk acoustic wave resonance structures comprise at least one firstbulk acoustic wave resonance structure 3, at least one second bulkacoustic wave resonance structure 3′ and at least one third bulkacoustic wave resonance structure 3″, wherein in the Step C5 comprisesfollowing steps of: the Step C51, the Step C52, the Step C53, the StepC54 and the Step C55. The structure of FIG. 5J is formed after the StepC51 and the Step C52, wherein the second polish layer 51 is polishedsuch that the at least one first sacrificial structure mesa 6, the atleast one second sacrificial structure mesa 6′ and the at least onethird sacrificial structure mesa 6″ are not exposed, thereby the secondpolish layer 51 under the polished surface 41 (the extending plane 43)and located under the at least one first bulk acoustic wave resonancestructure 3, the at least one second bulk acoustic wave resonancestructure 3′ and the at least one third bulk acoustic wave resonancestructure 3″ form a first frequency tuning structure 50 of the at leastone first bulk acoustic wave resonance structure 3, a second frequencytuning structure 50′ of the at least one second bulk acoustic waveresonance structure 3′ and a third frequency tuning structure 50″ of theat least one third bulk acoustic wave resonance structure 3″respectively; wherein the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ have a first thickness differenceTD1, the first thickness difference TD1 is equal to the first heightdifference HD1: wherein the first frequency tuning structure 50 and thethird frequency tuning structure 50″ have a second thickness differenceTD2, the second thickness difference TD2 is equal to the second heightdifference HD2. The structure of FIG. 5K is formed after the Step C53,the Step C54, the Step C55 and the Step C6, wherein the plural ofcavities comprise at least one first cavity 40, at least one secondcavity 40′ and at least one third cavity 40″; wherein the at least onefirst bulk acoustic wave resonance structure 3 and the at least onesecond bulk acoustic wave resonance structure 3′ have a first resonancefrequency difference FD1, the first resonance frequency difference FD1is correlated to the first thickness difference TD1 of the firstfrequency tuning structure 50 and the second frequency tuning structure50′; that is that the first resonance frequency difference FD1 iscorrelated to the first height difference HD1; thereby adjusting thefirst height difference HD1 is capable of tuning the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′; wherein the at least one first bulk acousticwave resonance structure 3 and the at least one third bulk acoustic waveresonance structure 3″ have a second resonance frequency difference FD2,the second resonance frequency difference FD2 is correlated to thesecond thickness difference TD2 of the first frequency tuning structure50 and the third frequency tuning structure 50″; that is that the secondresonance frequency difference FD2 is correlated to the second heightdifference HD2, thereby adjusting the second height difference HD2 iscapable of tuning a second resonance frequency difference FD2 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one third bulk acoustic wave resonance structure 3″; wherein thesecond polish layer 51 is made of insulator.

Please refer to FIGS. 5L and 5M, which are the cross-sectionalschematics showing steps of another embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. In current embodiment, the substrate 10 is acompound semiconductor substrate; the sacrificial structure 21 is madeof an epitaxial structure. The main steps for forming the embodimentshown in FIG. 5M are basically the same as those for forming theembodiment shown in FIG. 5K, except that in the Step C52, the secondpolish layer 51 is polished such that the at least one first sacrificialstructure mesa 6 is exposed and the at least one second sacrificialstructure mesa 6′ and the at least one third sacrificial structure mesa6″ are not exposed (please referring to FIG. 5L), thereby the secondpolish layer 51 under the polished surface 41 (the extending plane 43)and located under the at least one second bulk acoustic wave resonancestructure 3′ forms a second frequency tuning structure 50′ of the atleast one second bulk acoustic wave resonance structure 3′; the secondpolish layer 51 under the polished surface 41 (the extending plane 43)and located under the at least one third bulk acoustic wave resonancestructure 3″ forms a third frequency tuning structure 50″ of the atleast one third bulk acoustic wave resonance structure 3″. As shown inFIG. 5L, the second frequency tuning structure 50′ has a thickness T2,the thickness T2 of the second frequency tuning structure 50′ is equalto the first height difference HD1; wherein the third frequency tuningstructure 50″ has a thickness T3, the thickness T3 of the thirdfrequency tuning structure 50″ is equal to the second height differenceHD2. In current embodiment, there is no such a first frequency tuningstructure 50 as shown in FIG. 5K. Therefore, the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′ is correlated to the thickness T2 of the secondfrequency tuning structure 50′; that is that the first resonancefrequency difference FD1 is correlated to the first height differenceHD1 of the at least one first sacrificial structure mesa 6 and the atleast one second sacrificial structure mesa 6′; thereby adjusting thefirst height difference HD1 is capable of tuning the first resonancefrequency difference FD1 of the at least one first bulk acoustic waveresonance structure 3 and the at least one second bulk acoustic waveresonance structure 3′; the second resonance frequency difference FD2 ofthe at least one first bulk acoustic wave resonance structure 3 and theat least one third bulk acoustic wave resonance structure 3″ iscorrelated to the thickness T3 of the third frequency tuning structure50″; that is that the second resonance frequency difference FD2 iscorrelated to the second height difference HD2 of the at least one firstsacrificial structure mesa 6 and the at least one third sacrificialstructure mesa 6″; thereby adjusting the second height difference HD2 iscapable of tuning a second resonance frequency difference FD2 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one third bulk acoustic wave resonance structure 3″. In currentembodiment, the second polish layer 51 may be made of at least onematerial selected from the group consisting of: metal, alloy andinsulator.

Please refer to FIGS. 5N and 5O, which are the cross-sectionalschematics showing steps of another embodiment of a method of frequencytuning for bulk acoustic wave resonator of bulk acoustic wave filter ofthe present invention. In current embodiment, the substrate 10 is acompound semiconductor substrate; the sacrificial structure 21 is madeof an epitaxial structure. The main steps for forming the embodimentshown in FIG. 5O are basically the same as those for forming theembodiment shown in FIG. 5K, except that in the Step C5, the plural ofbulk acoustic wave resonance structures are formed on an extending plane43, wherein the extending plane 43 is coincident with the pre-polishedsurface 42, wherein the Step C5 comprises following steps of: Step C51′:forming a second polish layer 51 on the plural of sacrificial structuremesas and the insulating layer 11, wherein the second polish layer 51 ismade of at least one material selected from the group consisting of:metal and alloy; wherein in a preferable embodiment, the second polishlayer 51 is made of at least one material selected from the groupconsisting of: Ru, Ti, Mo, Pt, Au, Al and W; Step C52′: (pleasereferring to FIG. 5J) polishing the second polish layer 51 by achemical-mechanical planarization process to form a polished surface 41such that the plural of sacrificial structure mesas are not exposed;Step C53′: (please referring to FIG. 5N) patterning the second polishlayer 51; Step C54′: forming a piezoelectric layer 31 on the polishedsurface 41; and Step C55′: forming a top electrode layer 32 on thepiezoelectric layer 31. After the Step C6 etching the plural ofsacrificial structure mesas, the embodiment of FIG. 5O is formed. In theStep C4, the at least one second sacrificial structure mesa 6′ and theat least one third sacrificial structure mesa 6″ are etched; wherein thesecond polish layer 51 above the pre-polished surface 42 (the extendingplane 43), under the polished surface 41 and located under the at leastone first bulk acoustic wave resonance structure 3 forms a bottomelectrode layer 30 of the at least one first bulk acoustic waveresonance structure 3; wherein the second polish layer 51 above thepre-polished surface 42 (the extending plane 43), under the polishedsurface 41 and located under the at least one second bulk acoustic waveresonance structure 3′ forms a bottom electrode layer 30 of the at leastone second bulk acoustic wave resonance structure 3′; wherein the secondpolish layer 51 under the pre-polished surface 42 (the extending plane43) and located under the at least one second bulk acoustic waveresonance structure 3′ forms a second frequency tuning structure 50′ ofthe at least one second bulk acoustic wave resonance structure 3′;wherein the second frequency tuning structure 50′ has a thickness T2,the thickness T2 of the second frequency tuning structure 50′ is equalto the first height difference HD1; wherein the second polish layer 51above the pre-polished surface 42 (the extending plane 43), under thepolished surface 41 and located under the at least one third bulkacoustic wave resonance structure 3″ forms a bottom electrode layer 30of the at least one third bulk acoustic wave resonance structure 3″;wherein the second polish layer 51 under the pre-polished surface 42(the extending plane 43) and located under the at least one third bulkacoustic wave resonance structure 3″ forms a third frequency tuningstructure 50″ of the at least one third bulk acoustic wave resonancestructure 3″; the third frequency tuning structure 50″ has a thicknessT3, the thickness T3 of the third frequency tuning structure 50″ isequal to the second height difference HD2; thereby adjusting the firstheight difference HD1 is capable of tuning the first resonance frequencydifference FD1 of the at least one first bulk acoustic wave resonancestructure 3 and the at least one second bulk acoustic wave resonancestructure 3′; thereby adjusting the second height difference HD2 iscapable of tuning the second resonance frequency difference FD2 of theat least one first bulk acoustic wave resonance structure 3 and the atleast one third bulk acoustic wave resonance structure 3″.

Please refer to FIG. 5P, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.The main steps for forming the embodiment shown in FIG. 5P are basicallythe same as those for forming the embodiment shown in FIG. 5K, exceptthat the Step C5 comprises following steps of: Step C51″: forming asecond polish layer 51 on the plural of sacrificial structure mesas andthe insulating layer 11, wherein the substrate 10 is a compoundsemiconductor substrate; the plural of sacrificial structure mesas ismade of an epitaxial structure; wherein the second polish layer 51 ismade of at least one material selected from the group consisting of:metal, alloy and insulator; Step C52″: (please referring to FIG. 5J)polishing the second polish layer 51 by a chemical-mechanicalplanarization process to form a polished surface 41 such that the atleast one first sacrificial structure mesa 6, the at least one secondsacrificial structure mesa 6′ and the at least one third sacrificialstructure mesa 6″ are not exposed; Step C53″: (please referring to FIG.5N) patterning the second polish layer 51; Step C54″: forming a bottomelectrode layer 30 on the polished surface 41 (the extending plane 43);Step C55′: forming a piezoelectric layer 31 on the bottom electrodelayer 30; and Step C56″: forming a top electrode layer 32 on thepiezoelectric layer 31. After the Step C6, the embodiment of FIG. 5P isformed, wherein the second polish layer 51 under the polished surface 41(the extending plane 43) and located under the at least one first bulkacoustic wave resonance structure 3, the at least one second bulkacoustic wave resonance structure 3′ and the at least one third bulkacoustic wave resonance structure 3″ form a first frequency tuningstructure 50 of the at least one first bulk acoustic wave resonancestructure 3, a second frequency tuning structure 50′ of the at least onesecond bulk acoustic wave resonance structure 3′ and a third frequencytuning structure 50″ of the at least one third bulk acoustic waveresonance structure 3″ respectively; wherein the first frequency tuningstructure 50 and the second frequency tuning structure 50′ have a firstthickness difference TD1, the first thickness difference TD1 is equal tothe first height difference HD1; wherein the first frequency tuningstructure 50 and the third frequency tuning structure 50″ have a secondthickness difference TD2, the second thickness difference TD2 is equalto the second height difference HD2: thereby adjusting the first heightdifference HD1 is capable of tuning the first resonance frequencydifference FD1 of the at least one first bulk acoustic wave resonancestructure 3 and the at least one second bulk acoustic wave resonancestructure 3′; thereby adjusting the second height difference HD2 iscapable of tuning a second resonance frequency difference FD2 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one third bulk acoustic wave resonance structure 3″.

The embodiments of FIGS. 5K, 5M, 5O and 5P may also be formed from thestructure of FIG. 5A, wherein the substrate 10 may be a semiconductorsubstrate; the plural of sacrificial structure mesas may be made of atleast one material selected from the group consisting of: metal, alloyand epitaxial structure.

Please refer to FIGS. 6A˜6C, which are the cross-sectional schematicsshowing steps of an embodiment of a method of frequency tuning for bulkacoustic wave resonator of bulk acoustic wave filter of the presentinvention. In the embodiment, the substrate 10 may be a semiconductorsubstrate; the plural of sacrificial structure mesas may be made of atleast one material selected from the group consisting of: metal, alloyand epitaxial structure. The main structure of the epitaxial structureof the embodiment in FIG. 6A is basically the same as that of theembodiment in FIG. 3B, except that the plural of sacrificial structuremesas comprise at least one first sacrificial structure mesa 6, at leastone second sacrificial structure mesa 6′ and at least one thirdsacrificial structure mesa 6″. The main steps for forming the embodimentshown in FIG. 6C are basically the same as those for forming theembodiment shown in FIG. 4D, except that at least one first bulkacoustic wave resonator 1, at least one second bulk acoustic waveresonator 1′ and at least one third bulk acoustic wave resonator 1″ areformed on the substrate 10 respectively; wherein in the Step C, theplural of sacrificial structure mesas comprise at least one firstsacrificial structure mesa 6, at least one second sacrificial structuremesa 6′ and at least one third sacrificial structure mesa 6″; wherein inthe Step C4, the at least one first sacrificial structure mesa 6, the atleast one second sacrificial structure mesa 6′ and the at least onethird sacrificial structure mesa 6″ are etched such that the at leastone first sacrificial structure mesa 6 and the at least one secondsacrificial structure mesa 6′ have a first height difference HD1, andthe at least one first sacrificial structure mesa 6 and the at least onethird sacrificial structure mesa 6″ have a second height difference HD2(please referring to FIG. 6B); wherein in the Step C5, the plural ofbulk acoustic wave resonance structures comprise at least one first bulkacoustic wave resonance structure 3, at least one second bulk acousticwave resonance structure 3′ and at least one third bulk acoustic waveresonance structure 3″, wherein the at least one first bulk acousticwave resonance structure 3, the at least one second bulk acoustic waveresonance structure 3′ and the at least one third bulk acoustic waveresonance structure 3″ are located above the at least one firstsacrificial structure mesa 6, the at least one second sacrificialstructure mesa 6′ and the at least one third sacrificial structure mesa6″ respectively; wherein in the Step C52, the second polish layer 51 ispolished such that the at least one first sacrificial structure mesa 6,the at least one second sacrificial structure mesa 6′ and the at leastone third sacrificial structure mesa 6″ are not exposed, wherein thesecond polish layer 51 under the polished surface 41 (the extendingplane 43) and located under the at least one first bulk acoustic waveresonance structure 3, the at least one second bulk acoustic waveresonance structure 3′ and the at least one third bulk acoustic waveresonance structure 3″ form a first frequency tuning structure 50 of theat least one first bulk acoustic wave resonance structure 3, a secondfrequency tuning structure 50′ of the at least one second bulk acousticwave resonance structure 3′ and a third frequency tuning structure 50″of the at least one third bulk acoustic wave resonance structure 3″respectively; wherein the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ have a first thickness differenceTD1, the first thickness difference TD1 is equal to the first heightdifference HD1; wherein the first frequency tuning structure 50 and thethird frequency tuning structure 50″ have a second thickness differenceTD2, the second thickness difference TD2 is equal to the second heightdifference HD2: thereby adjusting the first height difference HD1 iscapable of tuning the first resonance frequency difference FD1 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′; therebyadjusting the second height difference HD2 is capable of tuning a secondresonance frequency difference FD2 of the at least one first bulkacoustic wave resonance structure 3 and the at least one third bulkacoustic wave resonance structure 3″, wherein the second polish layer 51is made of insulator.

Please refer to FIG. 6D, which is the cross-sectional schematic showingan embodiment of a method of frequency tuning for bulk acoustic waveresonator of bulk acoustic wave filter of the present invention. Incurrent embodiment, the substrate 10 may be a semiconductor substrate;the plural of sacrificial structure mesas may be made of at least onematerial selected from the group consisting of: metal, alloy andepitaxial structure. The main steps for forming the embodiment shown inFIG. 6D are basically the same as those for forming the embodiment shownin FIG. 6C, except that in the Step C52, the second polish layer 51 ispolished such that the polished surface 41 (the extending plane 43) iscoincident with the pre-polished surface 42 or such that the polishedsurface 41 is lower than the pre-polished surface 42, and wherein the atleast one first sacrificial structure mesa 6, the at least one secondsacrificial structure mesa 6′ and the at least one third sacrificialstructure mesa 6″ are not exposed. In current embodiment, the secondpolish layer 51 may be made of at least one material selected from thegroup consisting of: metal, alloy and insulator.

Please refer to FIG. 6E, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention. Incurrent embodiment, the substrate 10 may be a semiconductor substrate;the plural of sacrificial structure mesas may be made of at least onematerial selected from the group consisting of: metal, alloy andepitaxial structure. The main steps for forming the embodiment shown inFIG. 6E are basically the same as those for forming the embodiment shownin FIG. 6C, except that in the Step C5, the plural of bulk acoustic waveresonance structures are formed on an extending plane 43, wherein theextending plane 43 is coincident with the pre-polished surface 42;wherein the Step C5 comprises following steps of: Step C51′: forming asecond polish layer 51 on the plural of sacrificial structure mesas andthe insulating layer 11, wherein the second polish layer 51 is made ofat least one material selected from the group consisting of: metal andalloy; wherein in a preferable embodiment, the second polish layer 51 ismade of at least one material selected from the group consisting of: Ru,Ti, Mo, Pt, Au, Al and W; Step C52′: polishing the second polish layer51 by a chemical-mechanical planarization process to form a polishedsurface 41 such that the plural of sacrificial structure mesas are notexposed; Step C53′: patterning the second polish layer 51; Step C54′:forming a piezoelectric layer 31 on the polished surface 41; and StepC55′: forming a top electrode layer 32 on the piezoelectric layer 31.After the Step C6 etching the plural of sacrificial structure mesas, theembodiment of FIG. 6E is formed; wherein the second polish layer 51above the pre-polished surface 42 (the extending plane 43), under thepolished surface 41 and located under the at least one first bulkacoustic wave resonance structure 3 forms a bottom electrode layer 30 ofthe at least one first bulk acoustic wave resonance structure 3; whereinthe second polish layer 51 under the pre-polished surface 42 (theextending plane 43) and located under the at least one first bulkacoustic wave resonance structure 3 forms a first frequency tuningstructure 50 of the at least one first bulk acoustic wave resonancestructure 3; wherein the second polish layer 51 above the pre-polishedsurface 42 (the extending plane 43), under the polished surface 41 andlocated under the at least one second bulk acoustic wave resonancestructure 3′ forms a bottom electrode layer 30 of the at least onesecond bulk acoustic wave resonance structure 3′; wherein the secondpolish layer 51 under the pre-polished surface 42 (the extending plane43) and located under the at least one second bulk acoustic waveresonance structure 3′ forms a second frequency tuning structure 50′ ofthe at least one second bulk acoustic wave resonance structure 3′;wherein the first frequency tuning structure 50 and the second frequencytuning structure 50′ have a first thickness difference TD1, the firstthickness difference TD1 is equal to the first height difference HD1;wherein the second polish layer 51 above the pre-polished surface 42(the extending plane 43), under the polished surface 41 and locatedunder the at least one third bulk acoustic wave resonance structure 3″forms a bottom electrode layer 30 of the at least one third bulkacoustic wave resonance structure 3″; wherein the second polish layer 51under the pre-polished surface 42 (the extending plane 43) and locatedunder the at least one third bulk acoustic wave resonance structure 3″forms a third frequency tuning structure 50″ of the at least one thirdbulk acoustic wave resonance structure 3″; wherein the first frequencytuning structure 50 and the third frequency tuning structure 50″ have asecond thickness difference TD2, the second thickness difference TD2 isequal to the second height difference HD2: thereby adjusting the firstheight difference HD1 is capable of tuning the first resonance frequencydifference FD1 of the at least one first bulk acoustic wave resonancestructure 3 and the at least one second bulk acoustic wave resonancestructure 3′; thereby adjusting the second height difference HD2 iscapable of tuning a second resonance frequency difference FD2 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one third bulk acoustic wave resonance structure 3″.

Please refer to FIG. 6F, which is the cross-sectional schematic showinganother embodiment of a method of frequency tuning for bulk acousticwave resonator of bulk acoustic wave filter of the present invention.The main steps for forming the embodiment shown in FIG. 6F are basicallythe same as those for forming the embodiment shown in FIG. 6C, exceptthat the Step C5 comprises following steps of: Step C51″: forming asecond polish layer 51 on the plural of sacrificial structure mesas andthe insulating layer 11, wherein the substrate 10 is a compoundsemiconductor substrate; the plural of sacrificial structure mesas ismade of an epitaxial structure; wherein the second polish layer 51 ismade of at least one material selected from the group consisting of:metal, alloy and insulator; Step C52″: polishing the second polish layer51 by a chemical-mechanical planarization process to form a polishedsurface 41 such that the at least one first sacrificial structure mesa6, the at least one second sacrificial structure mesa 6′ and the atleast one third sacrificial structure mesa 6″ are not exposed; StepC53″: patterning the second polish layer 51; Step C54″: forming a bottomelectrode layer 30 on the polished surface 41 (the extending plane 43);Step C55″: forming a piezoelectric layer 31 on the bottom electrodelayer 30; and Step C56″: forming a top electrode layer 32 on thepiezoelectric layer 31. After the Step C6, the embodiment of FIG. 6F isformed, wherein the second polish layer 51 under the polished surface 41and located under the at least one first bulk acoustic wave resonancestructure 3, the at least one second bulk acoustic wave resonancestructure 3′ and the at least one third bulk acoustic wave resonancestructure 3″ form a first frequency tuning structure 50 of the at leastone first bulk acoustic wave resonance structure 3, a second frequencytuning structure 50′ of the at least one second bulk acoustic waveresonance structure 3′ and a third frequency tuning structure 50″ of theat least one third bulk acoustic wave resonance structure 3″respectively; wherein the first frequency tuning structure 50 and thesecond frequency tuning structure 50′ have a first thickness differenceTD1, the first thickness difference TD1 is equal to the first heightdifference HD1; wherein the first frequency tuning structure 50 and thethird frequency tuning structure 50″ have a second thickness differenceTD2, the second thickness difference TD2 is equal to the second heightdifference HD2; thereby adjusting the first height difference HD1 iscapable of tuning a first resonance frequency difference FD1 of the atleast one first bulk acoustic wave resonance structure 3 and the atleast one second bulk acoustic wave resonance structure 3′; therebyadjusting the second height difference HD2 is capable of tuning a secondresonance frequency difference FD2 of the at least one first bulkacoustic wave resonance structure 3 and the at least one third bulkacoustic wave resonance structure 3″.

The embodiments of FIGS. 6C, 6D, 6E and 6F may also be formed from theepitaxial structure of FIG. 5E, wherein the substrate 10 is a compoundsemiconductor substrate; the sacrificial structure 21 is made of anepitaxial structure.

The sacrificial structure 21 of FIG. 5E may further comprises a topetching stop layer 26 formed on the first fine tuning layer 23 (notshown in Figure), thereby the embodiments of FIGS. 6C, 6D, 6E and 6F maybe formed from the epitaxial structure of FIG. 5E, wherein the functionof the top etching stop layer 26 is basically the same as that of thetop etching stop layer 26 of the embodiment of FIG. 4J. In order toprevent from that the thickness of the first fine tuning layer 23 of theplural of sacrificial structure mesas located near the center of thesubstrate 10 is unequal to the thickness of the first fine tuning layer23 of the plural of sacrificial structure mesas located away from thecenter of the substrate 10, the top etching stop layer 26 is introducedsuch that the thickness of the first fine tuning layer 23 of the pluralof sacrificial structure mesas located near the center of the substrate10 is capable to be preserved to be equal to the thickness of the firstfine tuning layer 23 of the plural of sacrificial structure mesaslocated away from the center of the substrate 10.

The common characteristic of the embodiments of FIGS. 5C, 5K, 5P, 6C,6D, 6E and 6F is that: the bottom electrode layer 30 of the at least onefirst bulk acoustic wave resonance structure 3, the bottom electrodelayer 30 of the at least one second bulk acoustic wave resonancestructure 3′ and the bottom electrode layer 30 of the at least one thirdbulk acoustic wave resonance structure 3″ are all formed on theextending plane 43; and the first frequency tuning structure 50, thesecond frequency tuning structure 50′ and the third frequency tuningstructure 50″ are all formed under the extending plane 43. The commoncharacteristic of the embodiments of FIGS. 5D, 5M and 5O is that: thebottom electrode layer 30 of the at least one first bulk acoustic waveresonance structure 3, the bottom electrode layer 30 of the at least onesecond bulk acoustic wave resonance structure 3′ and the bottomelectrode layer 30 of the at least one third bulk acoustic waveresonance structure 3″ are all formed on the extending plane 43; and thesecond frequency tuning structure 50′ and the third frequency tuningstructure 50″ are all formed under the extending plane 43.

The embodiments of FIGS. 3G, 3I, 3K, 3L, 4D, 4F, 4H, 4I, 5C, 5D, 5K, 5M,5O, 5P 6C, 6D, 6E and 6F may further comprises a bottom etching stoplayer 20 (as the embodiment of FIG. 2I or FIG. 2J), wherein the bottometching stop layer 20 is formed on the substrate 10, the insulatinglayer 11 is formed on the bottom etching stop layer 20, wherein the atleast one first cavity 40 and the at least one second cavity 40′ (andthe at least one third cavity 40″) are located above the bottom etchingstop layer 20, wherein the substrate 10 is a compound semiconductorsubstrate; the plural of sacrificial structure mesas (the sacrificialstructure 21) may be made of an epitaxial structure.

Please refer to FIG. 6G, which is the partial enlarged cross-sectionalschematic showing an embodiment of a method of frequency tuning for bulkacoustic wave resonator of bulk acoustic wave filter of the presentinvention. The structure of FIG. 6G is the partial enlargedcross-sectional schematic showing one of the embodiments of FIGS. 1F,1K, 2F, 2I and 5C. The bottom electrode layer 30 of the at least onefirst bulk acoustic wave resonance structure 3 is very slowly andgradually thinned at the edge, so that the crystallization of thepiezoelectric layer 31 near the edge of the bottom electrode layer 30may maintain a good state without causing a phenomenon of crystal cracksor breakage. Therefore, in the structure of FIG. 6G, the bottomelectrode layer 30 very slowly and gradually thinned at the edge,therefore. FIG. 6G is a preferable embodiment. In other embodiments ofthe present invention, the bottom electrode layer 30 of the at least onefirst bulk acoustic wave resonance structure 3 (or the at least onesecond bulk acoustic wave resonance structure 3′, or the at least onethird bulk acoustic wave resonance structure 3″) may also have thesimilar structure that the bottom electrode layer 30 is very slowly andgradually thinned at the edge as that of the embodiment of FIG. 6G.Please refer to FIG. 6H, which is the partial enlarged cross-sectionalschematic showing another embodiment of a method of frequency tuning forbulk acoustic wave resonator of bulk acoustic wave filter of the presentinvention. The structure of FIG. 6H is the partial enlargedcross-sectional schematic showing one of the embodiments of FIGS. 3L and5P of the present invention. In the embodiment, besides the bottomelectrode layer 30 of the at least one first bulk acoustic waveresonance structure 3 is very slowly and gradually thinned at the edge,the second polish layer 51 is also very slowly and gradually thinned atthe edge. In the embodiments of FIGS. 4I and 6F of the presentinvention, the second polish layer 51 may be also very slowly andgradually thinned at the edge.

In the embodiments of the present invention, if the thickness of thefirst frequency tuning structure 50 (or the second frequency tuningstructure 50′, or the third frequency tuning structure 50″) is toothick, then it will affect the resonance state of the at least one firstbulk acoustic wave resonance structure 3 (or the at least one secondbulk acoustic wave resonance structure 3′, or the at least one thirdbulk acoustic wave resonance structure 3″). Therefore, the thickness ofthe first frequency tuning structure 50 (or the second frequency tuningstructure 50′, or the third frequency tuning structure 50″) is requiredto be less than 1000 nm. In some preferable embodiments, the thicknessof the first frequency tuning structure 50 (or the second frequencytuning structure 50′, or the third frequency tuning structure 50″) isequal to or less than 300 nm.

The present invention further provides a method for fabricating bulkacoustic wave resonator with mass adjustment structure. Please refer toFIGS. 7A˜7C, which show the cross-sectional schematics showing steps ofan embodiment of a method for fabricating bulk acoustic wave resonatorwith mass adjustment structure of the present invention. A method forfabricating bulk acoustic wave resonator 1 with mass adjustmentstructure of the present invention comprises following steps of: StepD1: forming a sacrificial structure mesa 6 on a substrate 10, whereinthe sacrificial structure mesa 6 is divided into a plurality of parts(7) including a central part 70 and a peripheral part 71, wherein theperipheral part 71 of the sacrificial structure mesa 6 is peripheralsurrounding the central part 70 of the sacrificial structure mesa 6,wherein the peripheral part 71 of the sacrificial structure mesa 6 has awidth X1; Step D2: etching the sacrificial structure mesa 6 such thatany two adjacent parts of the sacrificial structure mesa 6 havedifferent heights (the central part 70 of the sacrificial structure mesa6 and the peripheral part 71 of the sacrificial structure mesa 6 have aheight difference Y1), wherein a highest part (in current embodiment,the central part 70) of the sacrificial structure mesa 6 has a highestmesa top surface, wherein a mesa top extending plane 44 is coincidentwith the highest mesa top surface (please referring to FIG. 7A); StepD3: forming an insulating layer 11 on the sacrificial structure mesa 6and the substrate 10, wherein the insulating layer 11 is made of atleast one material selected from the group consisting of: siliconnitride (SiN_(x)), silicon oxide (SiO₂) and polymer; Step D4: polishingthe insulating layer 11 by a chemical-mechanical planarization processto form a polished surface 41 (please referring to FIG. 7B), wherein theplurality of parts (7) of the sacrificial structure mesa 6 has ageometric configuration; Step D5: forming a bulk acoustic wave resonancestructure 3 on the polished surface 41, wherein the bulk acoustic waveresonance structure 3 is located above the sacrificial structure mesa 6,wherein the Step D5 comprises following steps of: Step D51: forming abottom electrode layer 30 on the polished surface 41; Step D52: forminga piezoelectric layer 31 on the bottom electrode layer 30; and Step D53:forming a top electrode layer 32 on the piezoelectric layer 31; and StepD6: etching the sacrificial structure mesa 6 to form a cavity 40,wherein the cavity 40 is located under the bulk acoustic wave resonancestructure 3 (please referring to FIG. 7C); wherein, in the Step D4, theinsulating layer 11 is polished such that the sacrificial structure mesa6 is not exposed, wherein the insulating layer 11 under the bulkacoustic wave resonance structure 3, above the cavity 40, and betweenthe polished surface 41 and the mesa top extending plane 44 forms afrequency tuning structure, wherein the frequency tuning structure has athickness T, wherein the insulating layer 11 under the bulk acousticwave resonance structure 3 and between the mesa top extending plane 44and the cavity 40 forms a mass adjustment structure 8. In currentembodiment, the mass adjustment structure 8 comprises a peripheral massadjustment structure 81. The location of the peripheral mass adjustmentstructure 81 is corresponding to the peripheral part 71 of thesacrificial structure mesa 6. A width (X1) of the peripheral massadjustment structure 81 is equal to the width X1 of the peripheral part71 of the sacrificial structure mesa 6. A thickness (Y1) of theperipheral mass adjustment structure 81 is equal to the heightdifference Y1 of the central part 70 of the sacrificial structure mesa 6and the peripheral part 71 of the sacrificial structure mesa 6. Byforming the mass adjustment structure 8, the boundary condition of theperiphery of an acoustic resonance film which is formed by the topelectrode layer 32, the piezoelectric layer 31 and the bottom electrodelayer 30 of the bulk acoustic wave resonance structure 3 of the bulkacoustic wave resonator 1 is changed. Since the boundary condition ofthe periphery of the bulk acoustic wave resonance structure 3 ischanged, the ratio of the reflection acoustic wave and the incidentacoustic wave is also changed when the incident acoustic wave reflectsat the periphery of the bulk acoustic wave resonance structure 3. Thegeometric configuration of the plurality of parts (7) of the sacrificialstructure mesa 6 is correlated to a geometric configuration of the massadjustment structure 8; thereby by designing and adjusting a dimensionof the geometric configuration of the plurality of parts (7) of thesacrificial structure mesa 6, a dimension of the geometric configurationof the mass adjustment structure 8 is adjusted (in current embodiment,such as designing and adjusting the thickness Y1 or the width X1 of theperipheral mass adjustment structure 81), the ratio of the reflectionacoustic wave and the incident acoustic wave may be adjusted such thatthe Q factor of the bulk acoustic wave resonator 1 is effectivelyenhanced and, in the mean while, the spurious mode of the bulk acousticwave resonator 1 is suppressed. Furthermore, in current embodiment, theinsulating layer 11 may effectively enhance the mechanical structurestrength of the bulk acoustic wave resonance structure 3. Therefore, itmay avoid the bulk acoustic wave resonance structure 3 bendingdownwardly to touch the substrate 10 when affected by stress such thatthe characteristics of the bulk acoustic wave resonator 1 are affected.Moreover, enhancing the mechanical strength of the bulk acoustic waveresonance structure 3 of the bulk acoustic wave resonator 1 may avoidthe collapse of the bulk acoustic wave resonance structure 3 of the bulkacoustic wave resonator 1. In some embodiments, the substrate 10 is asemiconductor substrate, wherein the sacrificial structure mesa 6 ismade of at least one material selected from the group consisting of:metal, alloy and epitaxial structure.

In some other embodiments, the substrate 10 is a compound semiconductorsubstrate. The Step D1 comprises following steps of: Step D11: forming asacrificial structure 21 on the substrate 10; and Step D12: etching thesacrificial structure 21 to form the sacrificial structure mesa 6. Insome embodiments, the substrate 10 is made of GaAs; the sacrificialstructure comprises a GaAs layer. In some other embodiments, thesubstrate 10 is made of InP; the sacrificial structure comprises anInGaAs layer. In some embodiments, the sacrificial structure 21comprises a sacrificial epitaxial layer 27, a first etching stop layer22 and a first fine tuning layer 23, wherein the sacrificial epitaxiallayer 27 is formed on the substrate 10, the first etching stop layer 22is formed on the sacrificial epitaxial layer 27, the first fine tuninglayer 23 is formed on the first etching stop layer 22 (please referringto FIG. 2K). The height difference Y1 of the central part 70 of thesacrificial structure mesa 6 and the peripheral part 71 of thesacrificial structure mesa 6 is determined by a thickness of the firstfine tuning layer 23. Therefore, it helps to precisely adjust thethickness (Y1) of the peripheral mass adjustment structure 81, therebythe Q factor of the bulk acoustic wave resonator 1 is enhanced preciselyand, in the mean while, the spurious mode of the bulk acoustic waveresonator 1 is suppressed precisely.

Please refer to FIGS. 7D˜7E, which show the cross-sectional schematicsshowing steps of another embodiment of a method for fabricating bulkacoustic wave resonator with mass adjustment structure of the presentinvention. The main steps for forming the embodiment shown in FIG. 7Eare basically the same as the steps for forming the embodiment shown inFIG. 7C, except that, in the Step D4, the insulating layer 11 ispolished such that the sacrificial structure mesa 6 is exposed (pleasereferring to FIG. 7D), wherein the insulating layer 11 under the bulkacoustic wave resonance structure 3 and between the polished surface 41and the cavity 40 forms a mass adjustment structure 8 (please referringto FIG. 7E). The geometric configuration of the plurality of parts (7)of the sacrificial structure mesa 6 is correlated to a geometricconfiguration of the mass adjustment structure 8; thereby by adjustingthe geometric configuration of the plurality of parts (7) of thesacrificial structure mesa 6, the geometric configuration of the massadjustment structure 8 is adjusted such that a Q factor of the bulkacoustic wave resonator 1 is enhanced and, in the mean while, thespurious mode of the bulk acoustic wave resonator 1 is suppressed. Incurrent embodiment, the mass adjustment structure 8 comprises aperipheral mass adjustment structure 81. The location of the peripheralmass adjustment structure 81 is corresponding to the peripheral part 71of the sacrificial structure mesa 6. A width (X1) of the peripheral massadjustment structure 81 is equal to the width X1 of the peripheral part71 of the sacrificial structure mesa 6. The insulating layer 11 ispolished such that the sacrificial structure mesa 6 is exposed such thatthe polished surface 41 is coincident with the mesa top extending plane44 or the polished surface 41 is lower than the mesa top extending plane44, therefore a thickness (Y1′) of the peripheral mass adjustmentstructure 81 is equal to or less than the height difference Y1 of thecentral part 70 of the sacrificial structure mesa 6 and the peripheralpart 71 of the sacrificial structure mesa 6.

The present invention further provides a method for fabricating bulkacoustic wave resonator with mass adjustment structure. Please refer toFIGS. 7F˜7H, which show the cross-sectional schematics showing steps ofan embodiment of a method for fabricating bulk acoustic wave resonatorwith mass adjustment structure of the present invention. A method forfabricating bulk acoustic wave resonator 1 with mass adjustmentstructure of the present invention comprises following steps of: StepE1: forming a sacrificial structure mesa 6 on a substrate 10; Step E2:forming an insulating layer 11 on the sacrificial structure mesa 6 andthe substrate 10, wherein the insulating layer 11 is made of at leastone material selected from the group consisting of: silicon nitride(SiN_(x)), silicon oxide (SiO₂) and polymer; Step E3: polishing theinsulating layer 11 by a prior chemical-mechanical planarization processto form a pre-polished surface 42, such that the sacrificial structuremesa 6 is exposed, wherein the sacrificial structure mesa 6 is dividedinto a plurality of parts (7) including a central part 70 and aperipheral part 71, wherein the peripheral part 71 of the sacrificialstructure mesa 6 is peripheral surrounding the central part 70 of thesacrificial structure mesa 6, wherein the peripheral part 71 of thesacrificial structure mesa 6 has a width X1 (the Step E1, the Step E2and the Step E3 are similar to the steps showing in FIG. 3A and FIG. 3Bfor only one sacrificial structure mesa 6); Step E4: etching thesacrificial structure mesa 6 such that any two adjacent parts of thesacrificial structure mesa 6 have different heights (the central part 70of the sacrificial structure mesa 6 and the peripheral part 71 of thesacrificial structure mesa 6 have a height difference Y1), wherein ahighest part (in current embodiment, the central part 70) of thesacrificial structure mesa 6 has a highest mesa top surface, wherein amesa top extending plane 44 is coincident with the highest mesa topsurface, wherein the mesa top extending plane 44 is coincident with thepre-polished surface 42, wherein the plurality of parts (7) of thesacrificial structure mesa 6 has a geometric configuration (pleasereferring to FIG. 7F); Step E5: forming a bulk acoustic wave resonancestructure 3, wherein the bulk acoustic wave resonance structure 3 islocated above the sacrificial structure mesa 6, wherein the Step E5comprises following steps of: Step E51: forming a second polish layer 51on the sacrificial structure mesa 6 and the insulating layer 11, whereinthe second polish layer 51 is made of insulator; wherein in a preferableembodiment, the second polish layer 51 is made of at least one materialselected from the group consisting of: silicon nitride (SiN_(x)),silicon oxide (SiO₂), aluminum nitride (AlN) and polymer, wherein thepolymer may include Benzo Cyclobutane (BCB); Step E52: polishing thesecond polish layer 51 by a chemical-mechanical planarization process toform a polished surface 41 such that the sacrificial structure mesa 6 isnot exposed (please referring to FIG. 7G); Step E53: forming a bottomelectrode layer 30 on the polished surface 41; Step E54: forming apiezoelectric layer 31 on the bottom electrode layer 30; and Step E55:forming a top electrode layer 32 on the piezoelectric layer 31; and StepE6: etching the sacrificial structure mesa 6 to form a cavity 40 (pleasereferring to FIG. 7H), wherein the cavity 40 is located under the bulkacoustic wave resonance structure 3; wherein the second polish layer 51under the bulk acoustic wave resonance structure 3, above the cavity 40,and between the polished surface 41 and the mesa top extending plane 44forms a frequency tuning structure, wherein the frequency tuningstructure has a thickness T, wherein the second polish layer 51 underthe bulk acoustic wave resonance structure 3 and between the mesa topextending plane 44 and the cavity 40 forms a mass adjustment structure8. The geometric configuration of the plurality of parts (7) of thesacrificial structure mesa 6 is correlated to a geometric configurationof the mass adjustment structure 8; thereby by adjusting the geometricconfiguration of the plurality of parts (7) of the sacrificial structuremesa 6, the geometric configuration of the mass adjustment structure 8is adjusted such that a Q factor of the bulk acoustic wave resonator 1is enhanced and, in the mean while, the spurious mode of the bulkacoustic wave resonator 1 is suppressed. In current embodiment, the massadjustment structure 8 comprises a peripheral mass adjustment structure81. The location of the peripheral mass adjustment structure 81 iscorresponding to the peripheral part 71 of the sacrificial structuremesa 6. A width (X1) of the peripheral mass adjustment structure 81 isequal to the width X1 of the peripheral part 71 of the sacrificialstructure mesa 6. A thickness (Y1) of the peripheral mass adjustmentstructure 81 is equal to the height difference Y1 of the central part 70of the sacrificial structure mesa 6 and the peripheral part 71 of thesacrificial structure mesa 6. In some embodiments, the substrate 10 is asemiconductor substrate, wherein the sacrificial structure mesa 6 ismade of at least one material selected from the group consisting of:metal, alloy and epitaxial structure.

In some other embodiments, the substrate 10 is a compound semiconductorsubstrate. The Step E1 comprises following steps of: Step E11: forming asacrificial structure 21 on the substrate 10; and Step E12: etching thesacrificial structure 21 to form the sacrificial structure mesa 6. Insome embodiments, the substrate 10 is made of GaAs; the sacrificialstructure comprises a GaAs layer. In some other embodiments, thesubstrate 10 is made of InP; the sacrificial structure comprises anInGaAs layer. In some embodiments, the sacrificial structure 21comprises a sacrificial epitaxial layer 27, a first etching stop layer22 and a first fine tuning layer 23, wherein the sacrificial epitaxiallayer 27 is formed on the substrate 10, the first etching stop layer 22is formed on the sacrificial epitaxial layer 27, the first fine tuninglayer 23 is formed on the first etching stop layer 22 (please referringto FIG. 2K). The height difference Y1 of the central part 70 of thesacrificial structure mesa 6 and the peripheral part 71 of thesacrificial structure mesa 6 is determined by a thickness of the firstfine tuning layer 23. Therefore, it helps to precisely adjust thethickness (Y1) of the peripheral mass adjustment structure 81, therebythe Q factor of the bulk acoustic wave resonator 1 is enhanced preciselyand, in the mean while, the spurious mode of the bulk acoustic waveresonator 1 is suppressed precisely.

Please refer to FIGS. 7I˜7K, which show the cross-sectional schematicsshowing steps of another embodiment of a method for fabricating bulkacoustic wave resonator with mass adjustment structure of the presentinvention. The main steps for forming the embodiment shown in FIG. 7Hare basically the same as the steps for forming the embodiment shown inFIG. 7K, except that, in the Step E4, the sacrificial structure mesa 6is etched such that any two adjacent parts of the sacrificial structuremesa 6 have different heights (the central part 70 of the sacrificialstructure mesa 6 and the peripheral part 71 of the sacrificial structuremesa 6 have a height difference Y1), wherein a highest part (in currentembodiment, the central part 70) of the sacrificial structure mesa 6 hasa highest mesa top surface, wherein a mesa top extending plane 44 iscoincident with the highest mesa top surface, wherein the mesa topextending plane 44 is lower than the pre-polished surface 42, whereinthe plurality of parts (7) of the sacrificial structure mesa 6 has ageometric configuration (please referring to FIG. 7I). Please refer toFIG. 7J, which shows the result after the Step E52. After the Step E6,the second polish layer 51 under the bulk acoustic wave resonancestructure 3, above the cavity 40, and between the polished surface 41and the mesa top extending plane 44 forms a frequency tuning structure,wherein the frequency tuning structure has a thickness T, wherein thesecond polish layer 51 under the bulk acoustic wave resonance structure3 and between the mesa top extending plane 44 and the cavity 40 forms amass adjustment structure 8 (please referring to FIG. 7K).

The present invention further provides a method for fabricating bulkacoustic wave resonator with mass adjustment structure. Please refer toFIGS. 7L˜7M, which show the cross-sectional schematics showing steps ofan embodiment of a method for fabricating bulk acoustic wave resonatorwith mass adjustment structure of the present invention. A method forfabricating bulk acoustic wave resonator 1 with mass adjustmentstructure of the present invention comprises following steps of: StepF1: forming a sacrificial structure mesa 6 on a substrate 10; Step F2:forming an insulating layer 11 on the sacrificial structure mesa 6 andthe substrate 10, wherein the insulating layer 11 is made of at leastone material selected from the group consisting of: silicon nitride(SiN_(x)), silicon oxide (SiO₂) and polymer; Step F3: polishing theinsulating layer 11 by a prior chemical-mechanical planarization processto form a pre-polished surface 42, such that the sacrificial structuremesa 6 is exposed, wherein the sacrificial structure mesa 6 is dividedinto a plurality of parts (7) including a central part 70 and aperipheral part 71, wherein the peripheral part 71 of the sacrificialstructure mesa 6 is peripheral surrounding the central part 70 of thesacrificial structure mesa 6, wherein the peripheral part 71 of thesacrificial structure mesa 6 has a width X1 (the Step F1, the Step F2and the Step F3 are similar to the steps showing in FIG. 3A and FIG. 3Bfor only one sacrificial structure mesa 6); Step F4: etching thesacrificial structure mesa 6 such that any two adjacent parts of thesacrificial structure mesa 6 have different heights (the central part 70of the sacrificial structure mesa 6 and the peripheral part 71 of thesacrificial structure mesa 6 have a height difference Y1), wherein ahighest part (in current embodiment, the central part 70) of thesacrificial structure mesa 6 has a highest mesa top surface, wherein amesa top extending plane 44 is coincident with the highest mesa topsurface, wherein the mesa top extending plane 44 is coincident with thepre-polished surface 42, wherein the plurality of parts (7) of thesacrificial structure mesa 6 has a geometric configuration (pleasereferring to FIG. 7F); Step F5: forming a bulk acoustic wave resonancestructure 3, wherein the bulk acoustic wave resonance structure 3 islocated above the sacrificial structure mesa 6, wherein the Step F5comprises following steps of: Step F51: forming a second polish layer 51on the sacrificial structure mesa 6 and the insulating layer 11, whereinthe second polish layer 51 is made of at least one material selectedfrom the group consisting of: metal and alloy; wherein in a preferableembodiment, the second polish layer 51 is made of at least one materialselected from the group consisting of: Ru, Ti, Mo, Pt, Au, Al and W;Step F52: polishing the second polish layer 51 by a chemical-mechanicalplanarization process to form a polished surface 41 such that thesacrificial structure mesa 6 is not exposed (please referring to FIG.7G); Step F53: patterning the second polish layer 51 (please referringto FIG. 7L); Step F54: forming a piezoelectric layer 31 on the polishedsurface 41 of the second polish layer 51 and the pre-polished surface 42of the insulating layer 11; and Step F55: forming a top electrode layer32 on the piezoelectric layer 31; and Step F6: etching the sacrificialstructure mesa 6 to form a cavity 40, wherein the cavity 40 is locatedunder the bulk acoustic wave resonance structure 3; wherein the secondpolish layer 51 under the piezoelectric layer 31, above the cavity 40,and between the polished surface 41 and the mesa top extending plane 44forms a bottom electrode layer 30 of the bulk acoustic wave resonancestructure 3; wherein the second polish layer 51 under the bulk acousticwave resonance structure 3 and between the mesa top extending plane 44and the cavity 40 forms a mass adjustment structure (please referring toFIG. 7M). The geometric configuration of the plurality of parts (7) ofthe sacrificial structure mesa 6 is correlated to a geometricconfiguration of the mass adjustment structure 8; thereby by adjustingthe geometric configuration of the plurality of parts (7) of thesacrificial structure mesa 6, the geometric configuration of the massadjustment structure 8 is adjusted such that a Q factor of the bulkacoustic wave resonator 1 is enhanced and, in the mean while, thespurious mode of the bulk acoustic wave resonator 1 is suppressed. Incurrent embodiment, the mass adjustment structure 8 comprises aperipheral mass adjustment structure 81. The location of the peripheralmass adjustment structure 81 is corresponding to the peripheral part 71of the sacrificial structure mesa 6. A width (X1) of the peripheral massadjustment structure 81 is equal to the width X1 of the peripheral part71 of the sacrificial structure mesa 6. A thickness (Y1) of theperipheral mass adjustment structure 81 is equal to the heightdifference Y1 of the central part 70 of the sacrificial structure mesa 6and the peripheral part 71 of the sacrificial structure mesa 6. In someembodiments, the substrate 10 is a semiconductor substrate, wherein thesacrificial structure mesa 6 is made of at least one material selectedfrom the group consisting of: metal, alloy and epitaxial structure.

In some other embodiments, the substrate 10 is a compound semiconductorsubstrate. The Step F1 comprises following steps of: Step F11: forming asacrificial structure 21 on the substrate 10; and Step F12: etching thesacrificial structure 21 to form the sacrificial structure mesa 6. Insome embodiments, the substrate 10 is made of GaAs; the sacrificialstructure comprises a GaAs layer. In some other embodiments, thesubstrate 10 is made of InP; the sacrificial structure comprises anInGaAs layer. In some embodiments, the sacrificial structure 21comprises a sacrificial epitaxial layer 27, a first etching stop layer22 and a first fine tuning layer 23, wherein the sacrificial epitaxiallayer 27 is formed on the substrate 10, the first etching stop layer 22is formed on the sacrificial epitaxial layer 27, the first fine tuninglayer 23 is formed on the first etching stop layer 22 (please referringto FIG. 2K). The height difference Y1 of the central part 70 of thesacrificial structure mesa 6 and the peripheral part 71 of thesacrificial structure mesa 6 is determined by a thickness of the firstfine tuning layer 23. Therefore, it helps to precisely adjust thethickness (Y1) of the peripheral mass adjustment structure 81, therebythe Q factor of the bulk acoustic wave resonator 1 is enhanced preciselyand, in the mean while, the spurious mode of the bulk acoustic waveresonator 1 is suppressed precisely.

Please refer to FIGS. 7N˜7O, which show the cross-sectional schematicsshowing steps of another embodiment of a method for fabricating bulkacoustic wave resonator with mass adjustment structure of the presentinvention. The main steps for forming the embodiment shown in FIG. 7Oare basically the same as the steps for forming the embodiment shown inFIG. 7M, except that, in the Step F4, the sacrificial structure mesa 6is etched such that any two adjacent parts of the sacrificial structuremesa 6 have different heights (the central part 70 of the sacrificialstructure mesa 6 and the peripheral part 71 of the sacrificial structuremesa 6 have a height difference Y1), wherein a highest part (in currentembodiment, the central part 70) of the sacrificial structure mesa 6 hasa highest mesa top surface, wherein a mesa top extending plane 44 iscoincident with the highest mesa top surface, wherein the mesa topextending plane 44 is lower than the pre-polished surface 42, whereinthe plurality of parts (7) of the sacrificial structure mesa 6 has ageometric configuration (please referring to FIG. 7I). Please refer toFIG. 7J, which shows the result after the Step F52. Please refer to FIG.7N, which shows the result after the Step F53. After the Step F6, thesecond polish layer 51 under the piezoelectric layer 31, above thecavity 40, and between the polished surface 41 and the mesa topextending plane 44 forms a bottom electrode layer 30 of the bulkacoustic wave resonance structure 3; wherein the second polish layer 51under the bulk acoustic wave resonance structure 3 and between the mesatop extending plane 44 and the cavity 40 forms a mass adjustmentstructure (please referring to FIG. 7O).

Please refer to FIGS. 7P˜7Q, which show the cross-sectional schematicsshowing steps of another embodiment of a method for fabricating bulkacoustic wave resonator with mass adjustment structure of the presentinvention. The main steps for forming the embodiment shown in FIG. 7Qare basically the same as the steps for forming the embodiment shown inFIG. 7C, except that, in the Step D1, the plurality of parts (7)including a central part 70, a first peripheral part 71 and a secondperipheral part 72, wherein the second peripheral part 72 of thesacrificial structure mesa 6 is peripheral surrounding the central part70 of the sacrificial structure mesa 6, the first peripheral part 71 ofthe sacrificial structure mesa 6 is peripheral surrounding the secondperipheral part 72 of the sacrificial structure mesa 6, the secondperipheral part 72 of the sacrificial structure mesa 6 is locatedbetween the central part 70 of the sacrificial structure mesa 6 and thefirst peripheral part 71 of the sacrificial structure mesa 6, whereinthe first peripheral part 71 of the sacrificial structure mesa 6 has awidth X1, the second peripheral part 72 of the sacrificial structuremesa 6 has a width X2; in the Step D2, the sacrificial structure mesa 6is etched such that any two adjacent parts of the sacrificial structuremesa 6 have different heights (that is the two adjacent parts, thecentral part 70 of the sacrificial structure mesa 6 and the secondperipheral part 72 of the sacrificial structure mesa 6, have differentheights; and the two adjacent parts, the second peripheral part 72 ofthe sacrificial structure mesa 6 and the first peripheral part 71 of thesacrificial structure mesa 6, have different heights), wherein a highestpart (in current embodiment, the central part 70) of the sacrificialstructure mesa 6 has a highest mesa top surface, wherein a mesa topextending plane 44 is coincident with the highest mesa top surface,wherein the central part 70 (the highest part) of the sacrificialstructure mesa 6 and the second peripheral part 72 of the sacrificialstructure mesa 6 have a height difference Y2, the central part 70 (thehighest part) of the sacrificial structure mesa 6 and the firstperipheral part 71 of the sacrificial structure mesa have a heightdifference Y1, wherein the plurality of parts (7) of the sacrificialstructure mesa 6 has a geometric configuration (please referring to FIG.7P); wherein, in the Step D4, the insulating layer 11 is polished suchthat the sacrificial structure mesa 6 is not exposed, wherein theinsulating layer 11 under the bulk acoustic wave resonance structure 3,above the cavity 40, and between the polished surface 41 and the mesatop extending plane 44 forms a frequency tuning structure, wherein thefrequency tuning structure has a thickness T, wherein the insulatinglayer 11 under the bulk acoustic wave resonance structure 3 and betweenthe mesa top extending plane 44 and the cavity 40 forms a massadjustment structure 8 (please referring to FIG. 7P). The geometricconfiguration of the plurality of parts (7) of the sacrificial structuremesa 6 is correlated to a geometric configuration of the mass adjustmentstructure 8; thereby by adjusting the geometric configuration of theplurality of parts (7) of the sacrificial structure mesa 6, thegeometric configuration of the mass adjustment structure 8 is adjustedsuch that a Q factor of the bulk acoustic wave resonator 1 is enhancedand, in the mean while, the spurious mode of the bulk acoustic waveresonator 1 is suppressed. In current embodiment, the mass adjustmentstructure 8 comprises a first peripheral mass adjustment structure 81and a second peripheral mass adjustment structure 82. The location ofthe first peripheral mass adjustment structure 81 is corresponding tothe first peripheral part 71 of the sacrificial structure mesa 6, whilethe location of the second peripheral mass adjustment structure 82 iscorresponding to the second peripheral part 72 of the sacrificialstructure mesa 6. A width (X1) of the first peripheral mass adjustmentstructure 81 is equal to the width X1 of the first peripheral part 71 ofthe sacrificial structure mesa 6, while a width (X2) of the secondperipheral mass adjustment structure 82 is equal to the width X2 of thesecond peripheral part 72 of the sacrificial structure mesa 6. Athickness (Y1) of the first peripheral mass adjustment structure 81 isequal to the height difference Y1 of the central part 70 of thesacrificial structure mesa 6 and the first peripheral part 71 of thesacrificial structure mesa 6, while a thickness (Y2) of the secondperipheral mass adjustment structure 82 is equal to the heightdifference Y2 of the central part 70 of the sacrificial structure mesa 6and the second peripheral part 72 of the sacrificial structure mesa 6.In some embodiments, the substrate 10 is a semiconductor substrate,wherein the sacrificial structure mesa 6 is made of at least onematerial selected from the group consisting of: metal, alloy andepitaxial structure.

In some other embodiments, the substrate 10 is a compound semiconductorsubstrate. In some embodiments, the substrate 10 is made of GaAs; thesacrificial structure comprises a GaAs layer. In some other embodiments,the substrate 10 is made of InP; the sacrificial structure comprises anInGaAs layer. In some embodiments, the sacrificial structure 21comprises a sacrificial epitaxial layer 27, a second etching stop layer24, a second fine tuning layer 25, a first etching stop layer 22 and afirst fine tuning layer 23, wherein the sacrificial epitaxial layer 27is formed on the substrate 10, the second etching stop layer 24 isformed on the sacrificial epitaxial layer 27, the second fine tuninglayer 25 is formed on the second etching stop layer 24, the firstetching stop layer 22 is formed on the second fine tuning layer 25, thefirst fine tuning layer 23 is formed on the first etching stop layer 22(please referring to FIG. 5E). The height difference Y1 of the centralpart 70 of the sacrificial structure mesa 6 and the first peripheralpart 71 of the sacrificial structure mesa 6 is determined by a thicknessof the first fine tuning layer 23, a thickness of the first etching stoplayer 22 and a thickness of the second fine tuning layer 25; while theheight difference Y2 of the central part 70 of the sacrificial structuremesa 6 and the second peripheral part 72 of the sacrificial structuremesa 6 is determined by the thickness of the first fine tuning layer 23.Therefore, it helps to precisely adjust the thickness (Y1) of the firstperipheral mass adjustment structure 81 and the thickness (Y2) of thesecond peripheral mass adjustment structure 82, thereby the Q factor ofthe bulk acoustic wave resonator 1 is enhanced precisely and, in themean while, the spurious mode of the bulk acoustic wave resonator 1 issuppressed precisely.

In some embodiments, the first peripheral part 71 of the sacrificialstructure mesa 6 has a highest mesa top surface, wherein a mesa topextending plane 44 is coincident with the highest mesa top surface, thetwo adjacent parts, the central part 70 of the sacrificial structuremesa 6 and the second peripheral part 72 of the sacrificial structuremesa 6, have different heights; and the two adjacent parts, the secondperipheral part 72 of the sacrificial structure mesa 6 and the firstperipheral part 71 of the sacrificial structure mesa 6, have differentheights. The mass adjustment structure 8 comprises a central peripheralmass adjustment structure and a second peripheral mass adjustmentstructure 82 (not shown in Figure). In some other embodiments, thesecond peripheral part 72 of the sacrificial structure mesa 6 has ahighest mesa top surface, wherein a mesa top extending plane 44 iscoincident with the highest mesa top surface, the two adjacent parts,the central part 70 of the sacrificial structure mesa 6 and the secondperipheral part 72 of the sacrificial structure mesa 6, have differentheights; and the two adjacent parts, the second peripheral part 72 ofthe sacrificial structure mesa 6 and the first peripheral part 71 of thesacrificial structure mesa 6, have different heights. The massadjustment structure 8 comprises a central peripheral mass adjustmentstructure and a first peripheral mass adjustment structure 81 (not shownin Figure). In general, the mass adjustment structure 8 in theembodiments of FIGS. 7E, 7H, 7K, 7M and 7O may be similar to the massadjustment structure 8 in the embodiment of FIG. 7Q.

Please refer to FIG. 7R, which shows the cross-sectional schematicshowing an embodiment of a method for fabricating bulk acoustic waveresonator with mass adjustment structure of the present invention. Themain structure of the embodiment shown in FIG. 7R is basically the sameas the structure of the embodiment shown in FIG. 7C, except that abottom etching stop layer 12 is formed on the substrate 10, wherein theinsulating layer 11 is formed on the bottom etching stop layer 12. Inthe Step D6, the downward direction of the etching is stopped by thebottom etching stop layer 12. In some other embodiments, the substrate10 is a compound semiconductor substrate. In some embodiments, thesubstrate 10 is made of GaAs; the sacrificial structure comprises a GaAslayer; the bottom etching stop layer 12 is made of InGaP. In some otherembodiments, the substrate 10 is made of InP; the sacrificial structurecomprises an InGaAs layer; the bottom etching stop layer 12 is made ofInP. In general, the embodiments of FIGS. 7E, 7H, 7K. 7M, 7O and 7Q maycomprises a bottom etching stop layer 12 as shown in the embodiment ofFIG. 7R.

Please refer to FIG. 7S, which shows the electrode top-view shape of anembodiment of a method for fabricating bulk acoustic wave resonator withmass adjustment structure of the present invention. In the embodiment ofFIG. 7S, the main structure comprises a cavity 40, a bottom electrodelayer 30, a piezoelectric layer 31, a top electrode layer 32, a firstperipheral mass adjustment structure 81 and a second peripheral massadjustment structure 82. Along the section line a-a′ of FIG. 7S, across-sectional view is the embodiment of FIG. 7Q. The cavity 40 isunder the bottom electrode layer 30. The piezoelectric layer 31 isformed on the bottom electrode layer 30. The top electrode layer 32 isformed on the piezoelectric layer 31. The electrode top-view shape ofthe embodiment of FIG. 7S is an example of the shape of the bulkacoustic wave resonator with mass adjustment structure of the presentinvention. The shape of the bulk acoustic wave resonator with massadjustment structure of the present invention may be various. The topview (the shape) of the embodiments of FIG. 7C, 7E, 7H. 7K, 7M, 7O or 7Rmay be similar to FIG. 7S or may be various kinds of shapes.

A variation of a width of the top electrode layer 32 may also change theboundary condition of the periphery of the bulk acoustic wave resonancestructure 3. Therefore, the present invention may combine adjusting themass adjustment structure 8 and adjusting the width of the top electrodelayer 32 of the bulk acoustic wave resonance structure 3 such that the Qfactor of the bulk acoustic wave resonator 1 is effectively enhancedand, in the mean while, the spurious mode of the bulk acoustic waveresonator 1 is suppressed. In some embodiments, such as the embodimentsof FIGS. 7C, 7E, 7H, 7K, 7M, 7O, 7Q and 7R, the width of the topelectrode layer 32 of the bulk acoustic wave resonance structure 3 isequal to or less than a width of the cavity 40. In some otherembodiments, the width of the top electrode layer 32 of the bulkacoustic wave resonance structure 3 is equal to or less than a width ofthe central part 70 of the sacrificial structure mesa 6.

In some embodiments, the mass adjustment structure 8 may be made ofmetal material or insulator material. The metal material may be at leastone material selected from the group consisting of: Ti, Mo, Pt, A1, Au,W and Ru. The insulator material may be at least one material selectedfrom the group consisting of: silicon oxide, silicon nitride, aluminumnitride and polymer. The polymer may include Benzo Cyclobutane (BCB). Insome embodiments, the mass adjustment structure 8 may include thecombination of the materials mentioned above. For example, the massadjustment structure 8 may include the combination of the metalmaterials mentioned above.

In the embodiments of the present invention, the material of the bottomelectrode layer 30 may be at least one material selected from the groupconsisting of: Ti. Mo, Pt. A1, Au, W and Ru. The material of the topelectrode layer 32 may be at least one material selected from the groupconsisting of: Ti, Mo, Pt. A1, Au, W and Ru. In some embodiments of thepresent invention, the material of the piezoelectric layer 31 comprisesaluminum nitride. In some embodiments of the present invention, thematerial of the piezoelectric layer 31 comprises scandium doped aluminumnitride. In some other embodiments of the present invention, thematerial of the piezoelectric layer 31 comprises Zinc Oxide.

As disclosed in the above description and attached drawings, the presentinvention can provide a method for fabricating bulk acoustic waveresonator with mass adjustment structure. It is new and can be put intoindustrial use.

Although the embodiments of the present invention have been described indetail, many modifications and variations may be made by those skilledin the art from the teachings disclosed hereinabove. Therefore, itshould be understood that any modification and variation equivalent tothe spirit of the present invention be regarded to fall into the scopedefined by the appended claims.

What is claimed is:
 1. A method for fabricating bulk acoustic waveresonator with mass adjustment structure, comprising following steps of:Step D1: forming a sacrificial structure mesa on a substrate, whereinsaid sacrificial structure mesa is divided into a plurality of parts;Step D2: etching said sacrificial structure mesa such that any twoadjacent parts of said sacrificial structure mesa have differentheights, wherein a highest part of said sacrificial structure mesa has ahighest mesa top surface, wherein a mesa top extending plane iscoincident with said highest mesa top surface; Step D3: forming aninsulating layer on said sacrificial structure mesa and said substrate;Step D4: polishing said insulating layer by a chemical-mechanicalplanarization process to form a polished surface; Step D5: forming abulk acoustic wave resonance structure on said polished surface, whereinsaid bulk acoustic wave resonance structure is located above saidsacrificial structure mesa, wherein said Step D5 comprises followingsteps of: Step D51: forming a bottom electrode layer on said polishedsurface; Step D52: forming a piezoelectric layer on said bottomelectrode layer; and Step D53: forming a top electrode layer on saidpiezoelectric layer; and Step D6: etching said sacrificial structuremesa to form a cavity, wherein said cavity is located under said bulkacoustic wave resonance structure; wherein, in said Step D4, (1) saidinsulating layer is polished such that said sacrificial structure mesais not exposed, wherein said insulating layer under said bulk acousticwave resonance structure, above said cavity, and between said polishedsurface and said mesa top extending plane forms a frequency tuningstructure, wherein said insulating layer under said bulk acoustic waveresonance structure and between said mesa top extending plane and saidcavity forms a mass adjustment structure; or (2) said insulating layeris polished such that said sacrificial structure mesa is exposed,wherein said insulating layer under said bulk acoustic wave resonancestructure and between said polished surface and said cavity forms a massadjustment structure.
 2. The method for fabricating bulk acoustic waveresonator with mass adjustment structure according to claim 1, wherein,after said Step D4, said plurality of parts of said sacrificialstructure mesa has a geometric configuration; wherein said geometricconfiguration of said sacrificial structure mesa is correlated to ageometric configuration of said mass adjustment structure; thereby byadjusting said geometric configuration of said sacrificial structuremesa, said geometric configuration of said mass adjustment structure isadjusted such that a Q factor of said bulk acoustic wave resonator isenhanced.
 3. The method for fabricating bulk acoustic wave resonatorwith mass adjustment structure according to claim 1, wherein saidsubstrate is a semiconductor substrate; wherein said sacrificialstructure mesa is made of at least one material selected from the groupconsisting of: metal, alloy and epitaxial structure.
 4. The method forfabricating bulk acoustic wave resonator with mass adjustment structureaccording to claim 3, wherein said substrate is a compound semiconductorsubstrate; wherein said Step D1 comprises following steps of: Step D11:forming a sacrificial structure on said substrate; and Step D12: etchingsaid sacrificial structure to form said sacrificial structure mesa. 5.The method for fabricating bulk acoustic wave resonator with massadjustment structure according to claim 4, wherein (1) said substrate ismade of GaAs; said sacrificial structure comprises a GaAs layer; or (2)said substrate is made of InP; said sacrificial structure comprises anInGaAs layer.
 6. The method for fabricating bulk acoustic wave resonatorwith mass adjustment structure according to claim 5, further comprises afollowing step of: forming a bottom etching stop layer on saidsubstrate, wherein said sacrificial structure is formed on said bottometching stop layer; wherein (1) said bottom etching stop layer is madeof InGaP; or (2) said bottom etching stop layer is made of InP.